Last week a team of scientists working in Egypt announced a sensational discovery. They had detected a "void" roughly 100 feet wide and 26 feet high inside the Great Pyramid of Giza, built 4,500 years ago during the reign of Pharaoh Khufu.
It was the first time since the 19th Century that a major interior structure had been found within the pyramid, according to a paper about the discovery published in the journal Nature. And the discovery was made not by archaeologists wielding spades and brushes but by physicists using special detectors that "see" high-energy subatomic particles called muons.
Known as muon tomography, the method is likely now to be used to reveal long-hidden passageways, tombs, and chambers at archaeological sites around the world.
Tiny particles, big discoveries
Muons are created when cosmic rays from space slam into Earth's atmosphere at nearly the speed of light. These charged particles are like electrons, but with a greater mass, and they're constantly bombarding our planet's surface. One muon passes through an area the size of your fingernail each minute.
When muons pass through stone and other solid matter, they lose energy. Some come to a stop. Detectors strategically placed in various locations underneath a thick structure — in this case a pyramid some 460 feet tall — can be used to generate a three-dimensional map showing spots where muons are flying through empty space.
"You're making what looks like an X-ray," said Christopher Morris, a physicist at Los Alamos National Laboratory in New Mexico who was not involved in the discovery.
Archaeologists have a long history of using noninvasive imaging technologies. Ground-penetrating radar helps find unmarked graves, for example, and infrared cameras on drones reveal buried features via the temperature differences between stone walls and surrounding soil.
But when it comes to looking deep inside large structures, muon tomography can be the only option, said Arturo Menchaca-Rocha, a physicist at the National Autonomous University of Mexico who was not involved with the discovery. "Muons represent a highly penetrating type of natural radiation which is adequate to investigate large volumes, like a pyramid, even a volcano," Menchaca said.
Indeed, geologists use muon tomography to map changes occurring inside volcanoes before they erupt. Mining companies use it to map ore deposits deep underground. And now in archaeology, muon tomography is poised to reveal the secrets of large structures whose interiors are otherwise inaccessible.
There are other muographers at work on archaeological sites besides the Japanese-French team that investigated the Great Pyramid. Menchaca used muon tomography at the ancient city of Teotihuacan, near Mexico City — and discovered that the density differences within the 1,800-year-old Pyramid of the Sun there meant that the entire structure was at risk of collapsing like a dried-out sand castle.
Scientists working for the Maya Muon project at the University of Texas are looking for tombs and interior chambers in a previously unexplored pyramid in Belize. Morris's colleagues are using a variation of the technology called muon scattering — which Morris originally developed to detect nuclear weapons in cargo — to scan for cracks in the dome of the 600-year-old Cathedral of Santa Maria del Fiore in the Italian city of Florence.
Meanwhile, other physicists are busy identifying ancient sites that might be probed with muon tomography.
Italian physicists have proposed using muon detectors to search for hidden tombs at the ruins of the Punic-Roman site of Tharros in Sardinia. Physicists in France think the method could be useful at giant burial mounds known as tumuli. They created a model for how such a study could be done of the Amphipolis Tomb in Greece, which dates to the time of Alexander the Great and which caused a media frenzy when its entrance was discovered in 2014.
For all its promise, some archaeologists are wary of muon tomography — and not just because it's much costlier than other tools used in archaeology. Even the discovery of the void inside the Egyptian pyramid has its critics.
Mark Lehner, an expert on the pyramids at Giza, pointed out that the discovery revealed nothing about what might be inside the void. "I think it's not very probable that it's another chamber or burial with artifacts," Lehner said. "I think it's probably structural."
And David Anderson, an archaeologist at Radford University in Virginia, said he was dismayed by "media speculations on secret chambers, hidden treasures, and other more bizarre assertions" about the newly discovered void. He also believes the void is a structural feature designed to relieve stress on the Grand Gallery, the biggest of the Great Pyramid's inner chambers.
For now, archaeologists have no way to determine the function of the void. "You can't do what they did in the old days with gunpowder," Lehner said, a reference to the controlled explosions some British archaeologists used to get inside ancient structures during the 19th Century.
Looks like the next step will require something even more sophisticated than muon tomography.