A large void hidden deep within Khufu's Pyramid at Giza in Egypt has been discovered by a team of physicists. The first-ever image of the mysterious structure was taken using muons that shower down on Earth after being created when cosmic rays collide with the atmosphere.

The measurements were done by the ScanPyramids collaboration that includes researchers from Egypt, Japan and France. The team used three different muon-imaging techniques to study the pyramid, which was built in about 2500 BCE and is also known as the Great Pyramid and the Pyramid of Cheops.

Unexpected muons

Called muography, the technique is similar to radiography using X-rays. Dense materials such as stone tend to absorb muons, which travel relatively unhindered through the air. If more muons than expected reach a detector within the pyramid, it means that they must have passed through an air-filled void on their way.

In 2016 chemical-emulsion muon detectors developed at Nagoya University in Japan were deployed in the Queen's Chamber, which is the lowest known chamber within the pyramid (see figure). Much like photographic film, the emulsion undergoes a chemical reaction when exposed to muons. This leaves permanent 3D tracks in the detector that tell the researchers the directions from which the particles came.

As well as detecting known voids such as the King's Chamber, the emulsion detectors provided the first evidence for a previously-unknown large void about 30 m in length. "We knew we had found something very big and important," says Mehdi Tayoubi of the Heritage Innovation Preservation Institute and Dassault Systèmes – both in Paris.

To verify the existence of the void, scientists from the KEK particle physics lab in Japan installed hodoscopes at a separate location within the Queen's Chamber. These comprise layers of plastic scintillator, which measure muon trajectories. Outside the pyramid, physicists from France's nuclear research agency CEA monitored the muon flux through the pyramid using micromegas detectors. These were arranged in muon "telescopes", which are also able to measure muon trajectories.

We knew we had found something very big and important
Mehdi Tayoubi, Heritage Innovation Preservation Institute

Computer reconstruction

Using what are essentially three different 2D images taken from three different angles, the team could locate the void in 3D. A computer reconstruction based on analysis of the data suggests that it is similar to the Grand Gallery of the pyramid – which is an inclined passageway about 2 m wide, 8 m high and about 47 m long. The new void is between 50 and 70 m above ground level, which puts it above the Grand Gallery. The void is at about the same level as a series chambers that are above the King's Chamber – which lies near the centre of the pyramid. Tayoubi says that it is not clear whether the void is a single chamber or multiple chambers, or whether it is horizontal or inclined.

"The void is not predicted by any theory about the pyramid," says Tayoubi. He hopes that experts in ancient Egyptian architecture will be able to provide further information that could then be combined with the muon data in computer simulations in to determine what the void could be.

Very difficult to reach

He says that the location of the void would make it very difficult to reach by drilling and adds: “Our mission is non-destructive by design." However, he points out that the Nagoya team has also found a corridor-like structure near the surface of the pyramid that could provide a route to the newly discovered void.

This is not the first time that muons have been used to study the interior of pyramids. In the 1960s the American physicist and future Nobel laureate, Luis Alvarez, placed a muon detector in a chamber in the nearby Pyramid of Khafre. He showed that there are no other large chambers in that pyramid.

More recently, Arturo Menchaca of the National Autonomous University of Mexico placed a detector inside the Pyramid of the Sun at Teotihuacan near Mexico City. Physics World's James Dacey and Matin Durrani visited the experiment in 2015, where they recorded the podcast "Inside the particle pyramid". Dacey recounts how the intrepid pair crawled into the interior of the pyramid in "Particle-physics lab beneath a Mexican pyramid".

Elsewhere in Mexico, a team including Menchaca is trying to image the interior of a volcano using muons. See "Monitoring a smoking giant".