Secret chamber found in ancient pyramid: the Big Void and beyond

Arguably one of the most iconic legacies of ancient Egypt are the pyramids, which were built as tombs for pharaohs and other high-ranking officials and hundreds still stand, thousands of years later. Explorers have found their way inside many of them and uncovered rich archaeological treasure troves within. But could the ancient Egyptians have craftily concealed more inside their pyramids than we realise? At 140 metres high and 230 metres long, one of the largest pyramids has been well explored and studied, and the chambers inside have been thoroughly mapped. Or so we thought. Because now, using radiation streaming in from space to see through the pyramid, researchers have found something very exciting, as Eva Higginbotham heard from physicist Sébastien Procureur...

Eva - The Great Pyramid of Giza is thought to be the tomb of one of the earliest pharaohs of Egypt, King Khufu. More than 4,000 years after its construction, it's still receiving visitors.

Sébastien - It smells of dust, essentially. Surprisingly it's quite humid because there are many tourists visiting the place. There are even mice actually; we had some issues with the telescopes because mice were visiting them and eating some cables. There is some life inside!

Eva - That's Sébastien Procureur. He's a chief physicist on a project called Scan Pyramids. Launched in 2015, Scan Pyramids is a collaboration between institutions in Egypt, France, and Japan, all trying to gain a deeper understanding of the Great Pyramid.

Sébastien - The idea was to check the internal structure of a pyramid. We've done CT measurements with the idea to check whether there is any room, cavity, hole, corridor, or whatever, which could be inside the pyramid but which we've still not found.

Eva - Inside the pyramid, there are three main chambers that we know about: the King's Chamber, the Queen's Chamber, and an underground chamber that appears unfinished. There's also a 47 metre long corridor leading to the King's Chamber called the Grand Gallery. To investigate whether there were any other chambers inside the pyramid, the Scan Pyramids team used an imaging technique called 'cosmic muography'.

Sébastien - In photography you use a photon, which is light, to make an image of an object; but of course you have only the external image of the subject, you don't see inside. Muography, on the other hand, uses muon particles, and the advantage is that it can really see the inside of a structure.

Eva - It works by the detection of muons - elemental particles produced by the interaction between the atmosphere and other particles called cosmic rays. The muons make it down to earth and are highly penetrating. They can go through tens or hundreds of metres of solid rock, depending on their energy. Scientists can use the variable penetration of muons to figure out the density of a structure. It's like a race. At the start of a race you know how many people are taking part, and by looking at how many people make it to the finish line...

Sébastien - ...and just making a ratio of these two numbers, you have an estimate of the difficulty of the race. With muons it's exactly the same, that is, some of them muons will cross the whole pyramid, but some others will stop, just because they didn't have enough energy to cross the whole pyramid. So just by counting the number of muons crossing the pyramid in a given direction, you have an estimate of the difficulty of the race - that is, an estimate of thickness, and then the density.

Eva - The teams set up muon detectors inside and outside the pyramid at different locations. And by taking what are essentially cosmic muon photographs at different angles, they built up a picture of what the internal structure is like, including where some conspicuously empty areas were in what should have been solid rock.

Sébastien - With the muography measurements we were able to find three different cavities. The first cavity was found on the northeast edge; then a second cavity was found just on the north face, just behind what we call the chevron zone.

Sébastien - And the third cavity, which is the biggest, we've found above the Grand Gallery so it was called the Big Void. The length is estimated to be about 30 metres - between 30 and 40 metres long. So it's several hundreds of cubic metres in total

Eva - Sébastien and the team are confident that what they found is not just a deformation in the rock, or smaller stones that have crumbled, due to the careful way that they imaged the pyramid from multiple locations. They used simulations to help guide where they should put the detectors.

Sébastien - And it turned out that the position that the simulation gave us the best position was the - let's call it the restroom of the camels!. So we had to discuss with the camels...

Eva - And there's other evidence that what they found is a real secret chamber

Sébastien - What is also very intriguing and cannot be considered as a coincidence is that all of the structures I was talking about, so all the rooms and corridors, they are placed in the same plane in the pyramid. So it's a North-South plane, which is slightly shifted with respect to its centre, so of the plane is seven metres in the East direction. And it turns out that this Big Void is also placed in this special plane. So all this says that it cannot be a coincidence and there is really a big, significant void at this place.

Eva - I asked Sébastien if he thought there might be something inside the Big Void

Sébastien - If there is a tomb, if there is some object of any kind, even gold or whatever, I mean, we can imagine whatever! And actually there are already several theories of what this void can be. One theory is that it may be the real tomb of a pharaoh. Another theory is that it was used as a storage place. There's an interesting theory also which says it might be a second Grand Gallery. And in this theory actually the Grand Gallery was used to put the granite stones of the King's Chamber. Maybe a second Grand Gallery was required to put the highest stone of the King's Chamber.

Eva - It may be that the Big Void was created as a necessary part of the construction of the pyramid, and Sébastien is confident that future scans, with even newer techniques, will help reveal more details of how this great pyramid was built. Also, one of the long standing curiosities of Khufu's pyramid is that, unlike most other pyramids which are richly decorated with images and inscriptions, telling stories about the life of the person buried there, there is virtually nothing on the walls of Khufu's pyramid. On top of that, it's completely empty of the objects we know people were often buried with - things like pottery and gold that they would need in the afterlife. This is no surprise, really, as tomb robbery was absolutely rampant in ancient times, but still - perhaps the Big Void, and the other spaces they found, represent untouched, undisturbed chambers, which themselves could hold the key to understanding more about the mysterious Khufu's reign.