On the Giza Plateau in Egypt rise three large pyramids—the tallest and oldest of which is the Pyramid of Khufu. It is also known as simply the Great Pyramid of Giza. You know what it looks like. It’s one of the seven great wonders of the world.

Yet, for all its fame and antiquity, so many questions remain. How was it built? Why is there nothing in the pyramid, except a broken sarcophagus missing its lid? Could there be anything else hidden inside this massive structure? In the absence of information, there is of course ferocious speculation. And now, an intriguing new piece of information: the discovery, announced today, of a large, previously unknown “void” in the Great Pyramid.

This discovery comes by way of cosmic rays. When these high-energy rays hit atoms in the Earth’s atmosphere, they send subatomic particles called muons shooting toward the ground. The muons can be slowed down by large masses—like the rocks that make up the Great Pyramid. And if muons pass through a cavity inside a large mass, that cavity will show up on muon detectors, too. Three groups of particle physicists using three different techniques patiently tracked muon patterns over several months—gathering evidence that a large cavity lurked in the middle of the pyramid.

It is an incredible—and incredibly expensive—technical feat. ScanPyramids is a project of Cairo University and the Heritage Innovation Preservation (HIP) Institute, the latter of which is funded by a number of private technology and media companies.

As for what it all means, Egyptologists are being very cautious. “The significance of it is still an open question. Even the shape of the void is not quite clear yet,” says Peter Der Manuelian, an Egyptologist at Harvard University, who was not involved with the study.

In fact, the study’s authors exhorted journalists, please, please do not call it a secret chamber. “We know it is a void, but we don’t want to use the word ‘chamber,’” says Mehdi Tayoubi, president of the HIP Institute and an author on the paper. Their caution maybe sharpened by the reaction to a press release last October extolling their preliminary results, which media reports quickly indeed turned into speculation about “secret chambers.”

The void is above the Grand Gallery, though its exact shape and orientation is unknown. (Courtesy of ScanPyramids)

The new void is above the Grand Gallery—a passage with 28-foot vaulted ceilings leading to the King’s Chamber. The ScanPyramids group first saw hints of a void when they placed nuclear-emulsion film in the Queen’s Chamber, the room below the King’s Chamber. Nuclear-emulsion film records muons, not unlike how ordinary photographic film records photons. The team could see the Grand Gallery and the King’s Chamber in their muon pattern, but they also saw an anomaly. Two other teams of physicists—using instruments that detect muons passing through plastic arrays or argon—then verified this anomaly.

Using muons to study pyramids isn’t an entirely new idea. In the 1960s, future physics Nobel Prize winner Luis Alvarez took his early muon detector to the Pyramid of Khafre. He did not find any secret chambers or even unexpected anomalies. But the idea has lived on, and scientists have used muography to study volcanoes and man-made structures.

The nuclear-emulsion film setup in the Queen’s Chamber (Courtesy of ScanPyramids)
A setup for a muon telescope using argon gas in front of the north face of Khufu's Pyramid (Courtesy of ScanPyramids)

The ScanPyramids paper published in the scientific journal Nature is heavy on particle physics and deliberately light on archaeology. Hany Helal, an engineer at Cairo University and a member of the ScanPyramids team, says he is organizing a seminar in Egypt later this year, where archaeologists can come and debate the significance of the void for the pyramid’s construction.

Mark Lehner and Zahi Hawass, two members of the Egyptian Ministry of Antiquities’ scientific committee, to whom ScanPyramids presented it results earlier this year, both told me they suspected the void to be a “construction gap.” All of the chambers and major passageways of the pyramid are aligned along one vertical plane. In order to build the chambers and fill in the rest of the pyramid simultaneously, workers may have worked along what is essentially a trench that allowed them continual access to the King’s Chamber and Grand Gallery. A construction gap could be a remnant of the trench. So it is not surprising, they say, that a void from the construction gap might appear in the space above the Grand Gallery.

In contacting Egyptologists for this story, I could sense a weariness and wariness in their responses. Weariness because claims about hidden chambers in pyramids surface all the time.

The thing to understand, says Lehner, is “the pyramid is more Swiss cheese than cheddar.” That’s only a slight exaggeration, he adds. The inside of the Great Pyramid is filled with stones of irregular sizes, so there are numerous small gaps. In this case, he agrees the void appears to be large enough as to be deliberate, like a construction gap. But many people before have found evidence of a small cavity in the pyramid and gone on to speculate wildly about secret chambers. Lehner said he found ScanPyramids’ characterization of a different anomaly on the pyramid’s north face as a “corridor” to be premature.

The wariness, on the other hand, seems to stem from the project’s origins. Tayoubi, the president and cofounder of the HIP Institute, is also a VP at Dassault Systèmes, a French 3-D-design software company. In 2005, he teamed up to visualize the Great Pyramid construction site with architect Jean-Pierre Houdin, whose idea that the pyramids were built using a series of ramps is not accepted by mainstream archaeologists. (He has since also worked with Der Manuelian now at Harvard to reconstruct the Giza Plateau in 3-D.) Funding for the HIP Institute comes from a number of companies, including: Dassault, Japan’s national broadcasting agency, a watch company, a VR company, and a hotel near Giza.

Hawass, who is also a former Egyptian minister of antiquities, and an outsized, outspoken, and sometimes controversial figure in Egyptology, was blunt—his bluntness perhaps the result of longstanding frustration. “Everybody who comes to the pyramid,” he says, “either they’re looking for fame or they want to make experiments with their equipment and the equipment belongs to a company, and the company can make money.”

In an interview, Tayoubi acknowledged he is no Egyptologist, and he now assiduously avoided speculation on how the pyramid was built. He did want to tout the technologies used in the study, though not by company name. “We love innovation,” he says, “This mission is about better understanding the pyramid, but above all it’s about innovation,” he says. He likened studying the pyramid to space exploration—an endeavor driven by pure wonder that may nevertheless result in practical innovations in fields like muography and robotics. In fact, the ScanPyramids project is already designing its next piece of technology, a robot to explore inside the pyramid.

New technology might one day crack some of the questions about the Great Pyramid. But so much of its appeal may just be how little we know, despite its prominence and endurance. A mystery right in front of us, daring us to solve it.