The mysteries of the pyramids have deepened with the discovery of what appears to be a giant void within the Khufu, or Cheops, monument in Egypt.
It is not known why the cavity exists or indeed if it holds anything of value because it is not obviously accessible.
Japanese and French scientists made the announcement after two years of study at the famous pyramid complex.
They have been using a technique called muography, which can sense density changes inside large rock structures.
The Great Pyramid, or Khufu's Pyramid, was constructed during the reign of Pharaoh Khufu between 2509 and 2483 BC.
At 140m (460 feet) in height, it is the largest of the Egyptian pyramids located at Giza on the outskirts of Cairo.
Khufu famously contains three large interior chambers and a series of passageways, the most striking of which is the 47m-long, 8m-high Grand Gallery.
The newly identified feature is said to sit directly above this and have similar dimensions.
"We don't know whether this big void is horizontal or inclined; we don't know if this void is made by one structure or several successive structures," explained Mehdi Tayoubi from the HIP Institute, Paris.
"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."
The ScanPyramids team is being very careful not to describe the cavity as a "chamber".
Khufu contains compartments that experts believe may have been incorporated by the builders to avoid collapse by relieving some of the stress of the overlying weight of stone.
The higher King's Chamber, for example, has five such spaces above it.
The renowned American archaeologist Mark Lehner sits on a panel reviewing ScanPyramids' work.
He says the muon science is sound but he is not yet convinced the discovery has significance.
"It could be a kind of space that the builders left to protect the very narrow roof of the grand gallery from the weight of the pyramid," he told the BBC's Science In Action programme.
"Right now it's just a big difference; it's an anomaly. But we need more of a focus on it especially in a day and age when we can no longer go blasting our way through the pyramid with gunpowder as [British] Egyptologist Howard Vyse did in the early 1800s."
I'm sorry that reference is lost on me. I know who Al Capone was, but that's about it.