Egypt’s Great Pyramid of Giza – one of the wonders of the ancient world, and a dazzling feat of architectural genius – contains a hidden void at least a hundred feet long.

Scientists detected a previously unknown void in the largest of the pyramids in Giza, Egypt. They did it using subatomic particles called muons, according to findings published in the journal Nature, but the purpose of the void remains a mystery.

Archeologists already knew the pyramid, built for Egyptian pharoah Khufu more than 4,000 years ago, contained two burial chambers and a grand gallery. The newly discovered void, which scientists believe no modern person has entered, could be another room, but it could be something else. For example, ancient Egyptian architects sometimes built empty spaces into pyramids to take pressure off the structure, an Egyptologist mentioned in the journal’s study.

An archaeologist and Egyptologist Yukinori Kawae, a National Geographic Emerging Explorer, said, “This is definitely the discovery of the century.”