An invisible rain of tiny particles called muons pierces structures on Earth’s surface, including the Great Pyramid of Giza. Those muons can help map out the chambers within the pyramid. Some have even revealed a mysterious hidden void.

Inside Egypt’s Great Pyramid of Giza lies a mysterious cavity. This void has never been seen by any human alive today. Its surface is untouched by modern hands. Luckily, scientists are no longer limited to human senses.

To map out the pyramid’s unexplored interior, scientists tracked tiny particles called muons. Those subatomic particles are born high in Earth’s atmosphere.

From there, the muons hurtle toward the ground. Along the way, some have burrowed through the pyramid. Some of these particles left clues to their journey on sensitive detectors in and around the pyramid.

In 2017, the particles’ paths revealed the surprising presence of the hidden chamber.

That stunning find inspired physicists to explore other ancient structures the same way. The technique is now called muography (Mew-AW-gruh-fee). Some researchers are using it to map the inner plumbing of volcanoes. “You can see inside the volcano,” says Giovanni Leone. He’s a geophysicist at Universidad de Atacama. It’s in Copiapó, Chile. Such images could signal how and when a volcano is likely to erupt.

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Muons form when high-energy particles from space — cosmic rays — crash into Earth’s atmosphere. Their smashups high in Earth’s atmosphere create a constant shower of muons. They rain down at various angles everywhere on Earth’s surface. Scientists are now looking to use them to peer inside structures anywhere and everywhere.

When the muons reach Earth’s surface, they tickle the insides of large structures. Such as those pyramids. (They zip through smaller stuff too. Your thumbnail is pierced by a muon about once a minute.) Measuring how many muons something absorbs as they pass through it can reveal how dense the structure is. That, in turn, can expose any hidden gaps in the material.

The technique is sort of like taking a huge X-ray, explains Mariaelena D’Errico. But “instead of X-rays, we use … a natural source of particles.” That is, Earth’s very own, never-ending supply of muons. D’Errico is a particle physicist. She works at the National Institute for Nuclear Physics in Naples, Italy.

In the past, physicists studied cosmic rays to better understand outer space. But muography turns this tradition on its head. It uses these cosmic particles to learn more about concealed parts of our own world.

For the most part, “particles arriving from the universe have not been applied to our regular lives,” says Hiroyuki Tanaka. This particle physicist at the University of Tokyo and others are trying to change that.

Protons and other high-energy particles from space hit Earth’s atmosphere and produce a deluge of other particles. Pions and kaons can decay into muons, some of which reach Earth’s surface.

Muons are like the awkward cousins of electrons. Like electrons, they carry a negative electric charge. But muons are much heavier than electrons. And, unlike electrons, they don’t play a key role in atoms. In fact, when muons were first discovered, physicists wondered why these strange particles existed at all.

Muons, it turned out, are ideal for imaging the insides of large objects. A muon’s mass is about 207 times as large as an electron’s. That extra bulk means muons can pass through hundreds of meters of rock or more. If an electron passes through matter like a bullet, a muon tears through like a cannonball. A wall may stop a bullet, while a cannonball can pass through.

Another upside of muons: They are plentiful. They rain from the sky everywhere, all the time. So muon imaging needs no artificial radiation beam, such as the one produced by that X-ray machine in a doctor’s office. Muons “are for free,” says Cristina Cârloganu. This particle physicist works at CNRS and the National Institute of Nuclear and Particle Physics. She’s based in Aubière, France.

“They’re also very easy to detect,” says Richard Kouzes. He’s a nuclear physicist. He works at the Pacific Northwest National Laboratory in Richland, Wash. A simple detector made of plastic strips and light sensors can pick up muons. Other detectors need little more than special photographic film. Such instruments can detect both muons and their antiparticles. “Antimuons” are like muons, but carry a positive charge. They, too, shower down on Earth from high in the atmosphere.

When muons and antimuons pass through an object, they lose energy in various ways. One is by colliding with electrons in the material. That energy loss slows the particles. Sometimes they even stop. The denser the material, the fewer muons and antimuons that make it through to a detector under or next to the material.

Large, very dense objects — such as volcanoes or pyramids — will cast a muon shadow. Any gaps within those structures appear as bright spots within the shadow (because more particles slipped through). Inspecting such dappled shadows can open a vista into hidden worlds.

Muography first proved itself in a pyramid. In the 1960s, a team led by physicist Luis Alvarez hunted for hidden chambers in Khafre’s pyramid in Giza. This monument is a slightly smaller neighbor of the Great Pyramid. Detectors found no hint of unexpected rooms. The search did, however, prove the technique works.

Still, its use took didn’t take off right away. Muon detectors of the era tended to be bulky. And they worked best in well-controlled labs. To spot muons, Alvarez and his team used detectors called spark chambers. These chambers are filled with gas and metal plates under high voltage. When charged particles such as muons pass through, they create trails of sparks.

Today tech has largely replaced spark chambers. “We can make very compact, very sturdy detectors,” says Edmundo Garcia-Solis. He’s a nuclear physicist Chicago State University in Illinois. One type of detector that works outside the lab contains a type of chemical known as a scintillator. It emits light when a muon or other charged particle passes through it. Electronics then capture and measure that light.

Scientists placed three different types of muon detectors in and around the Great Pyramid of Giza. Data from these instruments helped the team map out the density of the pyramid and search for hidden chambers.

This year, physicists will use these detectors to take another look at Khafre’s pyramid. Kouzes and his colleagues announced their plan February 23 in the Journal for Advanced Instrumentation in Science. Their detector is small enough to fit inside two large carrying cases. Once inside the pyramid, it can be run with a laptop.

A nuclear emulsion film was key to finding the Great Pyramid’s hidden void in 2017. As muon detectors go, this muon detector is pretty low maintenance. It uses a special type of film to record the tracks as muons pass through. Researchers left detectors sitting in and around the pyramid. Later, they brought those films back to a lab to study the particle tracks they had recorded.

Kunihiro Morishima is a particle physicist at Nagoya University in Japan. He helped discover the Great Pyramid’s secret chamber. “Nuclear emulsions are lightweight, compact and do not require a power supply,” he says. That means multiple detectors can be placed at prime viewing sites. One of those was a room in the pyramid called the Queen’s Chamber. The team set up plastic scintillator detectors there, too. Meanwhile, gas-based detectors collected data from outside the pyramid.

Since discovering the void, Morishima and his colleagues have been busy taking more measurements. These data have helped sketch details of the void. The team placed emulsion detectors at 20 locations in the pyramid. They also put gas detectors at different spots. This new array of instruments revealed that the void is more than 40 meters (131 feet) long. The purpose of this space remains unknown.

Another group is planning a bigger survey of the Great Pyramid. Their idea is to place much larger detectors outside the pyramid. The detectors will be moved from place to place to measure incoming muons from multiple angles. This should give a 3-D view of what’s inside, says Alan Bross. He’s a particle physicist at Fermilab in Batavia, Ill. He is part of a team that shared its plans March 6 in the Journal for Advanced Instrumentation in Science.

Pyramids elsewhere in the world are also getting closer scrutiny. Garcia-Solis at Chicago State University and his colleagues plan to use muons to probe El Castillo. It’s a Maya pyramid at Chichén Itzá in Mexico. Morishima’s group plans to work on Maya pyramids, too. Scientists hope such studies might reveal new chambers or other hidden features.

There are other ways to peer inside objects. Radar, for example. Or ultrasound. Even X-rays. But each of these can probe only a short distance from the surface. Muons, in contrast, can paint an in-depth picture. For studying pyramids, Bross says, “muons really are ideal.”

Vesuvius is a known menace to Naples and surrounding areas in Italy. The volcano famously destroyed the ancient city of Pompeii in A.D. 79. Another major eruption in 1944 destroyed nearby villages. Since then, the volcano has been quiet. But if it erupted today, it could endanger the lives of some 600,000 people around it. Such a disaster might also threaten many others somewhat farther away.

“Vesuvius always scared me,” D’Errico says. “I was born and I live under this volcano.” Now, D’Errico is part of the Muon Radiography of Vesuvius experiment — MURAVES for short. Through this work, D’Errico seeks to better understand the volcano and its threat.

The team has set up muon detectors 1.5 kilometers (not quite a mile) from the volcano’s crater. This network is mapping muon densities — and thus rock densities — at the volcano’s top. It’s already turned up hints of density differences between the volcano’s northwestern and southeastern sides. The team shared that finding February 24 at arXiv.org.

MURAVES is still collecting data. Future observations should help scientists better understand the finer details of the volcano’s structure. The insides of Vesuvius are thought to be layered from repeated eruptions. Each new one could have piled new material atop the old.

Understanding a volcano’s structure could help predict what will happen when Vesuvius erupts next. For instance, it might help forecast where landslides could occur. Planning for those risks might help keep nearby people safe, says Cârloganu.

To see how, just look at Mount St. Helens in Washington. An entire flank of the volcano collapsed in a 1980 eruption. The disaster killed 57 people and caused widespread damage. Knowing where a volcano is weak could help predict how an eruption might play out, Cârloganu explains. And that could indicate what communities sit inside a danger zone.

Cârloganu has used muons to study a dormant volcano near Clermont-Ferrand, France. Now, she’s working to image Italy’s volcanic island of Vulcano. She thinks muons could point out this volcano’s weaknesses. But she doesn’t think they would warn when the volcano is going to blow.

Other researchers are more optimistic. Leone and Tanaka are among them. They wrote a paper on the topic last November. Muon imaging is ripe to include in volcano early-warning systems, they said. But it will take work, they noted in the Proceedings of the Royal Society A. Muon imaging has to be combined with other proven methods of eruption-forecasting, Leone says. Those include seismic measurements. Observations of ground motion and volcanic gases play a role, too.

Tanaka and colleagues are currently studying one of the most active volcanoes in the world. Called Sakurajima, it’s near Kagoshima, Japan. The volcano’s Showa crater erupted often until 2017. Then, the activity shifted abruptly to a different crater, Minamidake. By comparing muon data from before and after this happened, Tanaka’s team may have discovered why the shift occurred. A new, dense region had formed below the Showa crater. So Showa may have stopped erupting when a dense mass of solid magma plugged the crater.

These results suggest that scientists can use muons to help predict eruptions, Tanaka says. And so does another of his recent studies. Here, Tanaka and his colleagues fed muon data on the volcano’s structure into a deep-learning system. (Deep learning is a type of computer algorithm that can learn to make predictions from data.) Based on the muon data from a given week, the deep-learning system could predict whether the volcano would erupt the next day. The system correctly predicted eruption days more than 72 percent of the time. It correctly predicted non-eruption days more than 85 percent of the time.

Physics has incredible power to change how we see the world around us. The discovery of X-rays unveiled a whole new way to see hidden depths. Now, harnessing muons could change our perspective again. Science is starting to truy appreciate a particle once thought to be unnecessary. One day, in fact, these muons might save lives.

Superman’s X-ray vision was impressive. But some scientists might prefer muon vision. With closed shipping containers, for instance, muons could “see if there is something suspicious inside,” says Andrea Giammanco. This particle physicist works at the Université Catholique de Louvain in Belgium. One European project called Silent Border aims to develop a method to uncover dangerous, illegal cargo without having to open and inspect containers.

Key to this technique is measuring how muons scatter. When muons pass through a material, some are absorbed and some scatter, changing direction. Using detectors above and below an object, scientists can observe how a muon’s path changes as it passes through the object. Since muons tend to scatter away at larger angles when they hit materials made of heavier elements, this technique can reveal substances such as uranium.

Muon scattering measurements also could help peer into nuclear-waste storage containers to confirm what’s inside. This technique could rule out any theft of dangerous substances or other funny business.

Meanwhile, Giammanco and his colleagues are studying muon scattering for measuring strong magnetic fields. This could be useful for monitoring certain technologies that rely on magnetism. That includes experimental nuclear-fusion reactors — ones that scientists hope might one day power the planet. — Emily Conover

3-D: Short for three-dimensional. This term is an adjective for something that has features that can be described in three dimensions — height, width and length. 

algorithm: A group of rules or procedures for solving a problem in a series of steps. Algorithms are used in mathematics and in computer programs for figuring out solutions.

angle: The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

array: A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.

atmosphere: The envelope of gases surrounding Earth, another planet or a moon.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

cavity: (in geology or physics) A large rigid pocketlike structure.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague: Someone who works with another; a co-worker or team member.

cosmic rays: Very high-energy particles, mostly protons, that bombard Earth from all directions. These particles originate outside our solar system. They are equivalent to the nucleus of an atom. They travel through space at high rates of speed (often close to the speed of light).

crater: A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.

deep learning: A type of machine learning that is used by some computers. It relies on layers of neural networks, circuitry designed to function similarly to the human brain. Those networks sift through big sets of data to find trends. Each layer performs calculations and makes new predictions over and over. Then the system integrates what it’s learned to become “smarter” over time.

density: The measure of how condensed some object is, found by dividing its mass by its volume.

dormant: Inactive to the point where normal body functions are suspended or slowed down.

electric charge: The physical property responsible for electric force; it can be negative or positive.

electron: A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

electronics: Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.

emulsion: The mixture of a liquid and some other material (often fatty) that usually would not mix cleanly into the liquid. This can sometimes be accomplished by adding a special chemical (an emulsifier) or by shaking the two starting ingredients so that the one breaks up and at least for a while remains mixed with the other. Oil-based salad dressing can be one example of an emulsion.

eruption: (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.

factor: Something that plays a role in a particular condition or event; a contributor.

field: An area of study, as in: Her field of research is biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

fundamental: Something that is basic or serves as the foundation for another thing or idea.

ground-penetrating radar: A method of detecting the presence, position, distance or other important characteristics of objects (such as rocks or archeological artifacts) or substances (such as water or ice) underground. It works by sending out pulses of electromagnetic radiation that travel through earth and bounce off objects below. Scientists then measure how long it takes that bounced signal to return.

insight: The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.

magma: The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

matter: Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

Maya: A native American culture developed by people who lived between 2500 B.C. and 1500 A.D. in what is now parts of southern Mexico (its Yucatan Peninsula) and Central America. At its height (between about 250 and 900 A.D.), the density of people in some Mayan cities was equal to that in Medieval Europe.

metal: Something that conducts electricity well, tends to be shiny (reflective) and is malleable (meaning it can be reshaped with heat and not too much force or pressure).

muon: A type of unstable subatomic particle. Most on Earth formed when cosmic rays interact with atoms in the atmosphere. As leptons, muons belong to the same class of particles as the electron. Their mass, however, is roughly 200 times bigger. Muons tend to be short-lived. It tends to survive only 2.2 microseconds before it decaying (transforming) into an electron and two types of neutrinos.

network: A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)

Pacific: The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.

particle: A minute amount of something.

peer: (verb) To look into something, searching for details.

physics: The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

plastic: Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pyramid: A monumental structure with a square or triangular base and sloping sides that meet in a point at the top. The best known are those made from stone as royal tombs in ancient Egypt.

radar: A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.

sensor: A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly. (in biology) The structure that an organism uses to sense attributes of its environment, such as heat, winds, chemicals, moisture, trauma or an attack by predators.

solid: Firm and stable in shape; not liquid or gaseous.

subatomic: Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

system: A network of parts that together work to achieve some function. For instance, the blood, vessels and heart are primary components of the human body's circulatory system. Similarly, trains, platforms, tracks, roadway signals and overpasses are among the potential components of a nation's railway system. System can even be applied to the processes or ideas that are part of some method or ordered set of procedures for getting a task done.

tool: An object that a person or other animal makes or obtains and then uses to carry out some purpose such as reaching food, defending itself or grooming.

ultrasound: (adj. ultrasonic) Sounds at frequencies above the range that can be detected by the human ear. Also the name given to a medical procedure that uses ultrasound to “see” within the body.

universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

void: An empty space or cavity.

volcano: A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.

voltage: A force associated with an electric current that is measured in units known as volts. Power companies use high-voltage to move electric power over long distances.

Journal:​ M. D'Errico et al. The MURAVES experiment: study of the Vesuvius Great Cone with Muon Radiography. Posted February 24, 2022. arXiv:2202.12000.

Journal:​ A.D. Bross et al. Tomographic Muon Imaging of the Great Pyramid of Giza. Journal for Advanced Instrumentation in Science. Vol. 2022, March 6, 2022, p. 280. doi: 10.31526/jais.2022.280.

Journal:​ R.T. Kouzes et al. Novel Muon Tomography Detector for the Pyramids. Journal for Advanced Instrumentation in Science. Vol. 2022, February 23, 2022, p. 240.  doi: 10.31526/jais.2022.240.

Journal:​ K. Morishima et al. Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons. Nature. Vol. 552, November 2, 2017, p. 386. doi: 10.1038/nature24647.

Journal:​ Y. Nomura et al. Pilot study of eruption forecasting with muography using convolutional neural network. Scientific Reports. Vol. 10, March 24, 2020, p. 5272.  doi: 10.1038/s41598-020-62342-y.

Journal:​ L. Oláh et al. Plug formation imaged beneath the active craters of Sakurajima volcano with muography. Geophysical Research Letters. Vol. 46, September 6, 2019, p. 10417.  doi: 10.1029/2019GL084784.          

Meeting: M. D'Errico. The MURAVES experiment : study of the Vesuvius Great Cone with Muon Radiography. International Workshop on Cosmic-Ray Muography, November 25, 2021, Ghent, Belgium.

Journal:​ G. Leone et al. Muography as a new complementary tool in monitoring volcanic hazard: implications for early warning systems. Proceedings of the Royal Society A. Vol. 477, November 10, 2021, p. 20210320.  doi: 10.1098/rspa.2021.0320.

Journal:​ H.K.M. Tanaka et al. First results of undersea muography with the Tokyo-Bay Seafloor Hyper-Kilometric Submarine Deep Detector. Scientific Reports. Vol. 11, September 30, 2021, p. 19485.  doi: 10.1038/s41598-021-01979-9.

Science News physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

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