The Great Pyramid of Giza has been studied endlessly for hundreds of years, but it still hasn't given up all its secrets.
Scientists say they've discovered a hidden chamber about the size of a plane in the 4500-year-old wonder of the ancient world.
According to an article published in Nature, the "big void" is located deep within the pyramid, just above the Grand Gallery.
"It could be composed of one or several structures," said Mehdi Tayoubi, co-founder of the ScanPyramids project, which started work in 2015.
"Maybe it could be another Grand Gallery. It could be a chamber, it could be a lot of things. What we are sure about is that this big void is there, that it is impressive and that it was not expected as far as I know by any sort of theory."
It was found using cosmic-ray imaging, measuring the behaviour of subatomic particles blasted into the pyramid - similar to an X-ray, but much more powerful.
"The pyramid's burial chamber and sarcophagus have already been discovered, so this new area was more likely kept empty above the Grand Gallery to reduce the weight of stone pressing down on its ceiling," said Aidan Dodson, an Egyptologist at the University of Bristol.
It's not known if any objects are in the new gap - the technology is only precise enough to measure big, empty spaces.
Experts disagree over just how the pyramids were built, so the discovery of a massive void above the Great Gallery may finally confirm how it was put together. But it could be a while before its contents are revealed.
"We can no longer go blasting our way through the pyramid with gunpowder as Egyptologist Howard Vyse did in the early 1800s," archaeologist Mark Lehner told the BBC.
The Great Pyramid is the biggest of the three large pyramids found on the outskirts of Cairo, alongside the Sphinx.
It's the only one of the seven ancient wonders of the world still in existence, despite being by far the oldest.