No artificial structures on Earth have more mystery surrounding them than the Great Pyramids of Giza.

Believed to have been built roughly somewhere between 2550 and 2490BC, these magnificent pyramids are among the largest in the world.

Yet, despite being so well known, not much is known about what lies inside these towering structures.

Simple questions like exactly how they were built, and even why, continue to remain a source of debate among researchers. However, technology is finally on its way to uncovering some of the secrets of the dead within. What will they have to say?

A diagram showing the location in respect to the great chamber and the other known rooms in the Great Pyramid. Image: ScanPyramids 

What's known about the pyramids?

Located about 25km southwest of Cairo, the largest pyramid in the Giza Pyramid Complex (otherwise known as Giza Necropolis) stands 147m tall, and it alone took an estimated 20 to 30 years to build.

Known as the Pyramid of Khufu (after the Egyptian pharaoh Khufu – the second pharaoh of the fourth dynasty-who had it commissioned), it is taller than its neighbours: the pyramids of Menkaure and Khafre.

It is believed that all the pyramids were once covered in casing stones, made from a highly polished limestone, but over the millennia, most of the casing stones were pried loose and used on other building projects. Khafre's pyramid still has some of its limestone casing, but only at the very top.

Each pyramid in Giza was part of a temple complex which included a mortuary temple, a valley temple, and a series of sloping causeways linking them together. Smaller pyramids nearby became the final resting places of various members of the royal family, which leads us to...

A huge discovery

On November 26, 1922, archaeologists led by Howard Carter and Lord Carnarvon stumbled upon the mummified remains of King Tutankhamun – known as King Tut for short – in the Valley of the Kings in Luxor, about 515km south of Giza. The remains, like most of those found in the Valley of the Kings, were buried pyramids but beneath the sand not too far from the tomb of King Ramses VI.

They found his tomb and remains in pristine condition, incredible considering that the location of his tomb had been unknown for more than 3,000 years. 

At the time, most of the ancient Egyptian tombs had already been discovered, though the tomb of the little-known King Tutankhamen, who had ruled for a mere 10 years and died when he was 18, was still unaccounted for. The discovery of his mummified body and the thousands of priceless artefacts within the four-room tomb catapulted Carter – and Tut – into fame.

This discovery was one of the last major finds from Egypt's pharaonic period, and it was almost a century ago. But science can help shed more light on the pyramids and the culture of ancient Egypt perhaps?

Science could lead the way

There have been many different proposals for technology that could potentially help unearth the remaining mysteries of the Great Pyramids. In 2017, scientists announced that they have utilised particle physics to look inside of the pyramids, and they discovered a huge cavity that's more than 30m long.

The purpose of this void – known as ScanPyramids Big Void, is unknown, but its dimensions are very similar to the Grand Gallery, which leads to what was once the final resting place of Pharoh Khufu.

The technology used for this amazing discovery is called muon radiography, Essentially, scientists use muons, unstable subatomic particles similar to electrons, but with a much greater mass, to pierce through the pyramid's tough exterior and help map what lies inside.

Muons are generated when cosmic rays – high-energy particles from the Sun – collide with atoms in Earth's atmosphere. One useful quality of muons is their ability to penetrate into solid objects without affecting them, and it helps that they are literally everywhere. 

Starting in 2015, in order to map the pyramid, a physicist from Nagoya University, Kunihiro Morishima, placed several muon detectors inside the Queen’s Chamber, which is the lowest known chamber within the pyramid, and left them to collect data for several months.

Muons are highly penetrative and pass more easily through empty spaces than they do when they pass through solid objects. They lose energy but are not absorbed by the object – making them easy to detect.

In the case of the cavern in the Great Pyramid, he detected more muons than you would expect if they were travelling through a completely solid surface, which indicated that there was quite a bit of empty space deep in the pyramid’s interior.

Five-sigma level of statistical significance

Follow-up experiments in the following years, using different methods for recording the muons, confirmed the results and gave them at least a five-sigma level of statistical significance, meaning there is less than one-in-a-million chance the results are wrong.

Needless to say, there's little room to argue the results are a fluke – there is a mysterious cavern within the Great Pyramid. Although the reason for its existence remains unknown.

As well as detecting known voids such as the King’s Chamber, the muon detectors provided the first evidence for a previously unknown large void about 30m 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.

This technology has also been used to detect hidden tunnels and caverns beneath Mount Echia and peer inside the devastated remains of the Fukushima Daiichi nuclear power plant.

Moreover, archaeologists hope to use it to either prove or put to bed whether the rumoured aqueduct allegedly buried deep beneath the ancient Greek city of Cumae actually exists.

It also has practical uses in volcanology. Although unable to predict when a volcano will erupt when used on its own, scientists hope to someday to use the technology in combination with information gleaned through other sources to eventually predict when a volcano is going to blow.

How exactly can muons map the pyramids?

Three different kinds of muon detectors were utilised by researchers to confirm the existence of the Big Void: nuclear emulsion films, scintillator hodoscopes, and gas detectors. 

The nuclear emulsion films were first placed in the Queen's Chamber. Sort of like developing a picture, the films are placed beneath the thing you want to map – in this case, the upper chambers.

The muons travel from the atmosphere, pass through the pyramids, and are 'developed' like a long-exposure photo on the nuclear emission film. It's just as primitive as it sounds compared to newer technologies, but nuclear emulsion fields remain one of the most precise forms of muon measurement.

The seemingly empty region, which the researchers simply call 'the void', is thought to be at least 30m long.

However, its purpose remains unclear. Researchers have speculated that it may be leftover from the Great Pyramid’s construction, or may have been an internal ramp used to move the massive roof blocks of the King's Chamber into place.

Others suggest that the location of the void directly above the Grand Gallery suggests that it had a function to do with the gallery's construction. Researchers plan to continue to study the void, hoping to shed light on its purpose.

Scientists and archaeologists are always looking at ways in which we can use technology to shed light on the mysteries of the pyramids. Perhaps someday, we will be able to learn exactly how these amazing structures were built. 

This article was written by  and first appeared in Interesting Engineering.