Borrowing a technique from particle physics, an international team of researchers has harnessed cosmic-ray collisions to peek inside and uncover the hidden 100-foot “void” within the pyramid’s stones.

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The Great Pyramid of Giza has towered over Egypt for more than 4,500 years. Built during the reign of Pharaoh Khufu, the monument was a testament to the ruler’s architectural prowess and is thought to have been a home for his mummified remains.

For centuries, archaeologists have ventured into the Pyramid of Khufu, as it is also known, and marveled at the King’s chamber, the Queen’s chamber and the Grand Gallery. Now, using a technique from the field of particle physics, an international team of researchers has harnessed cosmic-ray collisions to peek inside and uncover a hidden “void” within the pyramid’s stones that is roughly 100 feet long, similar to the Statue of Liberty from her heel to her head.

“We don’t know if it’s a chamber, a tunnel, a big gallery or things like that,” said Mehdi Tayoubi, co-director of the ScanPyramids project, which published the finding last month in the journal Nature. “We have chosen the word ‘void’ and nothing else because we don’t know what this void is.”

Many archaeologists questioned whether the study offered any new information about the ancient Egyptians and were quick to note that the team most likely did not find a hidden room filled with the pharaoh’s riches. They said the so-called void was probably empty space designed by the pyramid’s architects to lessen the weight on its chambers and prevent them from collapsing, an example of features that were already documented in the construction of the ancient monuments.

However, the study may suggest that advances in technology can offer a richer understanding of wonders of the ancient world that have long fascinated the human imagination.

Khufu, also known by his Greek name Cheops, is thought to have ruled from 2509 B.C. to 2483 B.C., during Egypt’s fourth dynasty. Though he constructed the largest pyramid Egypt has ever seen, the only intact three-dimensional figure of him that archaeologists have found measures a mere 3 inches tall. Very little is known about him, so his pyramid offers one of the few looks into his life and reign. The site at Giza where his pyramid was built also contains two other major pyramids and the Sphinx.

Since 2015, Tayoubi and his colleagues, now consisting of three separate teams of physicists and engineers, have investigated the pyramid using a particle-physics technique known as muon tomography to see through to its core.

“We tried to do for the pyramid what a doctor can do with X-rays,” Tayoubi said.

Instead of X-rays, the team used muons, the heavy cousins of electrons that form when cosmic rays from outer space collide with particles in Earth’s atmosphere. The fallout from the collisions creates a constant bombardment of harmless particles that can penetrate deep into the planet. As the muons pass through matter they lose energy and decay, so if the team detected a small amount of muons, that means they were passing through matter. But if they detected more muons, it suggests the particles were passing through empty space or less dense material.

The technology was previously used by Luis Alvarez, a Nobel Prize-winning physicist, to investigate whether there were hidden chambers in the Pyramid of Khafre in the 1960s. As muon detector resolution has greatly improved over the decades, it has since been used to see the inner structures of volcanoes as well as the irradiated Fukushima nuclear reactor.

In 2016, Tayoubi’s colleagues stood in the Queen’s chamber and used muon detectors capable of making improved measurements to study particles as they passed through the pyramid. When they analyzed their data from a region above the Grand Gallery, a long inclined passageway that leads to the King’s Chamber, they found something strange: an unexpected excess of muons.

They found a void.

The first measurements were made by researchers from Nagoya University in Japan who were a part of the project. Then two more teams associated with ScanPyramids, one from France and another from Japan, also confirmed the anomaly with muon tomography, even from outside the pyramid. The discovery comes on the footsteps of the team’s previous work, which detected a small void behind the north face of the pyramid in 2016.

Christopher Morris, a physicist who conducts research using muon tomography at Los Alamos National Laboratory and was not involved in the study, called the findings “pretty amazing,” adding that all the team needed to do was set up the muon detectors and reap the rewards.

“All the other physicists who could have done it, and didn’t, are jealous,” he said.

But archaeologists were more critical of the work.

Mark Lehner, an Egyptologist from the Ancient Egypt Research Associates, said that previous work had shown that the ancient Egyptians most likely constructed gaps in their pyramids and that the voids the team found are nothing special, or new.

“The great pyramid of Khufu is more Swiss cheese than cheddar,” he said. He added that the steep incline of the void also casts doubts on whether it was some sort of room. “At that angle, it doesn’t make much sense for it to be a chamber that would contain artifacts, burials and objects and that sort of thing.”

Hany Helal, who is also co-director of the ScanPyramids project, responded to the criticism, saying that from an engineering perspective, it would not make sense to have such a big void above the Gallery if its purpose was to relieve pressure.

He said the next steps are to have an international discussion with archaeologists to figure out the structure’s purpose. In the future, he added that scientists may use drones to explore the void once they have more information about it.