Scientists have found a secret chamber inside the Great Pyramid. And that's just the start.

The discovery, the first of its kind since the 19th century, looks set to shed light on how the pyramids were built and help answer the still unexplained question of why exactly they were. But for the moment it deepens the mystery, leading to even more puzzles about how the strange structures came to exist.

The scientists announced their find as part of the ScanPyramids project, which aims to use cutting edge techniques to find out more about the structures. Previous announcements have included strange warm stones inside the pyramids, and other mysterious chambers.

What have they found?

A hole. But a very important one.

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The gap – which is being referred to as a chamber, but is really just a void that doesn't have solid stone within it – sits inside the pyramid and hasn't been found until now. But very little is known beyond that, and it is a discovery that leads to a lot more questions than it could possibly answer.

What's inside the void?

Nobody knows. All we know for certain is that there's some kind of hole, that it's about the size of a plane, that it doesn't appear to be connected to other passages inside the pyramid, and that it sits above the Grand Gallery.

Scientists aren't sure what the chamber was put there for, what it might look like, or what it contains. It might not even contain anything.

While some have guessed that it could be an ancient burial chamber or other similarly ceremonial, sacred place, others have said that it might be a much more practical addition.

The placing within the pyramid has led some to think that it could have been a ramp that was used to carry stones up to the higher bits of the pyramid. Though that's not as obviously exciting, it could still be incredibly important – it would be the first discovery of its kind and shed some light on how exactly the pyramids were built, a feat that still mystifies experts given its complexity and difficulty.

How did they find it?

The chamber was discovered using a cosmic particle detector. And that's exactly as futuristic and astounding as it sounds.

The precise technique is called "muon scanning", just one of the many advanced tools that the ScanPyramids team have at their disposal. By watching for the particles as they fall from the atmosphere, scientists can work out whether areas are empty or solid.

More specifically, that works by putting down special plates inside, under and around the pyramids. The particles pass through open spaces fine, but get caught up in solid parts – that affects the amount of them that can be found on the plates.

Scientists can then use that information to work out what parts of the inside of the pyramid have holes inside them. By using multiple plates and triangulating where the holes seem to be, they can build up a picture of where is solid and where's not inside the structure – allowing them pinpoint the size and place of the mysterious chamber.

What will they do now?

Try and find out what's inside. But if it's not already clear from the amount of work needed to find it, that won't be an easy job.

One way of trying to do that is to compare it to the kind of chambers we do know about. If there are other voids that we've been inside of or had a look at that have the same sort of shape and size, then that might offer a clue to what the new mysterious hole might have inside it, or at least what it could have been intended to be used for.