The Great Pyramid of Giza is hiding a huge, unexplored space — and scientists used cosmic rays to find it

More than 4,500 years after their construction, the pyramids of Giza continue to hide deep and ancient secrets.

A team of more than three dozen researchers announced on Thursday that they had discovered a huge, unexplored void in the pharaoh Khufu's pyramid, also called the Great Pyramid — the largest of three main structures in Giza, Egypt.

What's more, the team used cosmic rays from deep space to see through millions of tons of rock, locate the void, and estimate its size. They've determined that the cavernous space is roughly 100 feet long — about the length of two semitrailers — and sits almost directly above the Grand Gallery, a towering passage that leads to Khufu's tomb.

"This void was hidden in the construction of the pyramid," Mehdi Tayoubi, a leader of the research effort, called ScanPyramids, told reporters on Wednesday. "It is not accessible, and we needed this new technique, at the right time, to identify it and to discover it."

The research group published its findings Thursday in the journal Nature.

"A lot of people tried to dig some tunnels looking for chambers," said Tayoubi, who's also a founder of the HIP Institute, one of the organizations that designed the ScanPyramids mission. "But as far as I know, no one has tried to dig something in this area. There was no theory expecting to find something as big as the Grand Gallery here."

Tayoubi and his colleagues say they're 99.9999% certain that the "big void" exists, and they assert that it's neither an anomaly in the data nor porous rock or loose rubble.

Mark Lehner, an archaeologist and Egyptologist at the Ancient Egypt Research Association, agrees with that conclusion. He served on an Egypt-based advisory committee that oversaw the ScanPyramids project but wasn't involved in the work.

"I don't think this is bulls---," he told Business Insider. "I put credence in the results, and I think they have indications of a large, empty space."

How to probe a pyramid with cosmic rays

Pyramids are confounding objects to study because they are so large and so revered. It's tough to see how they're constructed — and what they're hiding — without boring, digging, or blasting away their ancient, stony features.

"In the past, it was easy: You have a scientist, and he says, 'I see a hidden door here,' and he can go and destroy this door and see what is behind it," Hany Helal, a ScanPyramids team member who's a pyramid expert at Cairo University, said during a press briefing. "Now, no one can allow for trial and error."

Khufu's pyramid is an especially difficult case. The 6.5-million-ton, 455-foot-tall monument is the largest and most ancient Egyptian pyramid, and its hidden internal structure is labyrinthine. Plus, it's also a prized national heirloom.

The pyramid's construction of high and deeply embedded tunnels and chambers, like its Grand Gallery, is also exceptional compared with other similar structures, Lehner said.

"In the popular imagination, it's the classic pyramid — but really, it's the anomaly," he added. "Nobody knew what Khufu did, and no one did it afterward."

However, the ScanPyramids team realized that a nondestructive particle-physics technology called muography, which is used to peer inside volcanoes, could be used to probe structures like Khufu's pyramid.

Muons are fast-moving, short-lived elementary particles that constantly rain down from the sky. They're made when cosmic rays from supernovas, merging neutron stars, black holes, and other high-energy objects reach Earth and pummel air molecules.

This creates a shower of particles that includes muons — thousands of which pass through our bodies every minute.

Muons move through air unhindered yet get weakly absorbed or deflected by rock.

So with special muon-detecting film and devices inside a pyramid, cavities will show up as bright spots as the rock absorbs fewer muons and more make it to the detectors and film.

Inside the Great Pyramid, just 1% of muons reach inner chambers.

That's why it took ScanPyramids two years to round up enough data — it takes months to expose a piece of film or run a detector, and the team used three techniques to show that the void exists.

Nothing to see here?

Many details about the void are still unknown — including its exact height and angle within the pyramid — so the group is wary about labeling it a room or a chamber.

"For the moment, it seems very difficult to access this very big void," Tayoubi said, adding that more research will be needed to define its shape and extent without destroying any internal masonry.

Helal and others would ultimately like to bore a small hole into slabs of rock to reach the void, pop in a tiny yet capable robot, and explore the space remotely.

Lehner is not sure anything other than secrets about the structure of the pyramid will be found inside, though. He thinks the void probably does not hold any undiscovered mummies, golden talismans, or other ancient artifacts. Lehner told Business Insider that Khufu's father laid the engineering groundwork for the pyramids but that it was under Khufu's reign that their construction was pioneered — and experimented with.

"Nobody is assessing the reality of the fabric of the pyramid itself," Lehner said. "This pyramid looks very regular from a distance, but it was made with a considerable slop factor."

For example, he said, there are many "relieving structures," including extra ceilings and spaces stacked above the King's Chamber close to the pyramid's center. Lehner and others think ancient Egyptians overengineered these sections as insurance policies against structural collapse — a risk due to the mass of the pyramid and shoddy internal construction that used backfill of rubble and sand.

"For all the world, I'll bet this is a relieving space for the Grand Gallery," he said, noting that this feature exists right below the void. "They probably thought they needed some kind of relieving space to separate the slop factor from the fine masonry."

Put another way: The big void may be nothing more than a stone-roof attic that protects a magnificent attraction several stories below, which means it's unlikely to house any ancient treasures.

ScanPyramids did not say how much its two-year effort cost, but Lehner suspects the project was expensive. Though he commends the discovery, he suggested such resources may be put to better use.

"Years ago, I realized I had to turn my back to the pyramids to understand them," Lehner said. "To understand the pyramids, we need to understand how ancient Egypt worked — their economy, where they lived, what they ate, what drove them."

Lehner said the void's discovery was not really a "wow moment." More shocking, he said, was the simple pyramid-construction logbook archaeologists recently found. Such documents reveal more about the pyramids than the monuments themselves, he said, and they're located only through traditional and relatively inexpensive archaeological fieldwork — not high-tech particle physics.

"Would the people of a 3,000-year civilization be better served by finding a hole," he said, "or a year's worth of their version of Business Insider?"