Cosmic Rays Reveal a Large Hidden Void in the Great Pyramid

Originally published 2017-11-18.

Cut-away view of Khufu’s pyramid. The hazy white points mark where the muon detectors located the new void. (Image: ScanPyramids mission)

Yes, there is a large, previously unknown empty space inside the Great Pyramid of Giza, and it was found without moving a single stone. In November 2017 an international team called ScanPyramids reported in the journal Nature that particles from cosmic rays, called muons, had revealed a cavity at least 30 metres (about 100 feet) long sitting directly above the Grand Gallery, the pyramid's famous sloping hall.

What nobody can yet say is what the space is for. It might be a sealed chamber, a corridor, a gap left to take weight off the gallery's roof, or something no one has thought of. The team named it simply the ScanPyramids Big Void. Six years later the same project went on to confirm a second, smaller hidden corridor near the entrance, this time seen directly with a camera.

This article explains how the void was found, why muons can see through stone, what experts think the space might be, and what has been confirmed since.

What the scientists actually found

ScanPyramids was launched in October 2015 by Cairo University and the French HIP Institute, with the approval of Egypt's antiquities authorities. Its goal was to scan four Old Kingdom pyramids for hidden structures using techniques that cause no damage. The 2017 paper was led by Kunihiro Morishima of Nagoya University and Mehdi Tayoubi of the HIP Institute.

Cross-section showing the position of the Big Void above the Grand Gallery. It is drawn horizontal here, but it may slope, possibly towards the north face. (Image: ScanPyramids mission)

Their results showed a space with a cross-section similar to the Grand Gallery's and a length of at least 30 metres. It lies roughly 21 metres above the pyramid's base, but its exact shape and angle are uncertain: it could be one long horizontal space, an inclined passage, or several smaller cavities close together. Crucially, three separate teams using three different detector technologies all saw the same signal.

How cosmic rays can see inside stone

Muons are heavy cousins of the electron. They are created high in the atmosphere when cosmic rays from space smash into air molecules, and they rain down on the Earth's surface constantly, at close to the speed of light. They are harmless: roughly one passes through an area the size of your hand every second.

The key property is that muons can travel through rock, but some are absorbed along the way. The more solid material they cross, the fewer make it out the other side. Place a detector inside or beside a pyramid and count the muons arriving from each direction over several months. Where more arrive than expected, there must be less stone in the way, and therefore some kind of empty space. The technique is called muography.

Inside the Grand Gallery. The Big Void sits directly above this hall and appears to be of a similar size and shape. (Image: ScanPyramids mission)

The idea is not new. In the late 1960s the American physicist Luis Alvarez, later a Nobel laureate, used muon detectors in the Belzoni Chamber of Khafre's pyramid, the second-largest at Giza. His team found no hidden rooms in the part of the pyramid it surveyed. Better detectors have since turned muography into a practical tool. It has been used to image magma inside volcanoes such as Vesuvius and Etna, and to locate melted fuel inside the damaged reactors at Fukushima Daiichi.

Three teams, three methods, one result

To make sure the signal was not an error in a single instrument, the project used three independent approaches:

• Nagoya University placed nuclear emulsion plates in the Queen's Chamber. These are like photographic film that records the track of every particle that passes through.
• KEK, Japan's High Energy Accelerator Research Organization, installed electronic scintillator detectors, which flash when a muon crosses them, also in the Queen's Chamber.
• CEA, the French Alternative Energies and Atomic Energy Commission, set up gas-based muon telescopes outside the pyramid, looking through it from the north face.

A Nagoya University researcher setting up a muon detector inside the Queen’s Chamber. (Image: ScanPyramids mission)

Because the muon flux is low and only a small fraction makes it through so much limestone, each team had to collect data for months; the first exposures began in December 2015. All three saw an excess of muons from the same region above the Grand Gallery. The team also compared the signal with the one produced by the Grand Gallery itself, a known open space, and found the two looked alike. That made a tiny crack, loose fill or "Swiss cheese" of small gaps far less likely, since muography only picks up large voids.

The pyramid and its known rooms

The Great Pyramid was built for the pharaoh Khufu (Cheops in Greek) of Egypt's Fourth Dynasty, around 2600 BC. Exact reign dates are debated, and most modern estimates place Khufu and his pyramid between about 2700 and 2500 BC. It originally stood about 146.6 metres (481 feet) tall and remained the tallest human-made structure on Earth for more than 3,700 years, until taller cathedral spires rose in medieval Europe.

Inside, the known spaces are the King's Chamber with its granite sarcophagus, the misleadingly named Queen's Chamber, an unfinished chamber cut into the bedrock about 27 metres below the base, and the Grand Gallery. The gallery is about 46.7 metres long and 8.6 metres high, with corbelled walls that step inward towards the ceiling. The original entrance is on the north face, but visitors today use a tunnel traditionally credited to the Caliph al-Ma'mun around AD 820, although some scholars think robbers dug it much earlier.

No plans of the pyramid survive. The oldest written evidence, the papyri found at Wadi al-Jarf in 2013 and known as the Diary of Merer, describes boats carrying limestone blocks from the Tura quarries to the building site in the 27th year of Khufu's reign. They tell us about logistics, not about the inside of the monument.

What could the void be?

The ScanPyramids team was careful not to call it a chamber. They argued that it looked deliberate rather than accidental, but that more observations from new angles were needed to understand its shape and purpose.

Team members studying a 3D reconstruction of the void using augmented reality. (Image: ScanPyramids mission)

Egyptologists offered several possibilities. One is a construction feature: a space left above the Grand Gallery to relieve the weight on its roof, similar to the stacked relieving chambers above the King's Chamber. Another is a working ramp or passage used during building and later sealed. UCLA Egyptologist Kara Cooney, who was not part of the study, pointed out that Khufu and his father Sneferu had begun placing chambers high inside their pyramids, so a hidden room is not out of the question. She added that finding grave goods almost 5,000 years old would be extraordinary.

Not everyone was impressed. Egypt's best-known archaeologist, Zahi Hawass, said the 2017 paper offered nothing new to Egyptology, and the antiquities ministry felt the results had been published too quickly. Others noted that many builders' gaps are known in pyramids. The debate is a useful reminder that "void" describes a measurement, not a treasure room.

Common myths

"They found a secret burial chamber." No room or contents have been seen; only an area of lower density.

"Muons are dangerous radiation." They are part of the natural background that reaches every person every day; the detectors simply count them.

"Researchers drilled into the pyramid." The 2017 discovery was entirely non-invasive. Drilling would require approval from Egypt's Supreme Council of Antiquities, which tightly protects the monument.

The hidden corridor confirmed in 2023

In 2016 the project had already spotted signs of a smaller cavity behind the chevron-shaped stones above the original entrance on the north face. In March 2023 the team published new muon results and Egyptian officials showed the first images from inside. A 6 mm endoscope from Japan, threaded through a small gap between stones, revealed an unfinished corridor roughly 9 metres long, with a gabled roof, lying about 7 metres from the entrance tourists use today.

Officials suggested it may have been built to spread the weight of the masonry above the entrance, and said scanning would continue. Officials described the corridor as the first major internal structure found in the pyramid since the 19th century. The far larger Big Void, however, has still not been reached or seen directly. Ideas floated for exploring it include tiny robots, one of them designed by Jean-Baptiste Mouret's team at the French research institute Inria, that could squeeze through a hole only a few centimetres wide.

Aerial view of the Great Pyramid of Khufu on the Giza Plateau. (Image: ScanPyramids mission)

For now, the oldest of the Seven Wonders of the Ancient World still keeps one of its secrets. What has changed is how we look for answers: with physics borrowed from particle accelerators, patient months of data and, for the first time, without harming the stones themselves.

Sources: Morishima et al., Nature (2017): Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons; Wikipedia: ScanPyramids; Wikipedia: Great Pyramid of Giza; Wikipedia: Muography; Inquirer/AFP (March 2023): hidden corridor revealed. Images courtesy of the ScanPyramids mission.