Physicists using cosmic rays detectors have located what appears to be an empty space inside the Great Pyramid in Egypt that has never been entered.

To see through the Great Pyramid, the researchers used a technique developed in high-energy particle physics: they tracked particles called muons, which are produced when cosmic rays strike atoms in the upper atmosphere. Around 10,000 muons rain down on each square metre of Earth’s surface every minute. Sensitive muon detectors have been developed for use in particle accelerators, but they have also been used in the past decade or so to determine the inner structures of volcanoes and to study the damaged nuclear reactor at Fukushima, Japan.

In December 2015, physicist Kunihiro Morishima of Nagoya University, Japan, and his colleagues placed a series of detectors inside the Queen’s chamber, where they would detect muons passing through the pyramid from above. The particles are partially absorbed by stone, so any large holes in the pyramid would result in more muons than expected hitting the detectors.

After several months, “we had an unexpected line”, says Tayoubi. To check the result, two other teams of physicists, from the Japanese High Energy Accelerator Research Organization in Tsukuba and the French Alternative Energies and Atomic Energy Commision in Paris, then used different types of muon detector placed in other locations both inside and outside the pyramid. All three teams observed a large, unexpected void in the same location above the Grand Gallery. …The space is at least 30 metres long, with a similar cross section to the Grand Gallery. “It was a big surprise,” says Tayoubi. “We’re really excited.”

It is unclear how or even if they will access this void. Right now its purpose remains a mystery, including whether it contains any artifacts.