Egypt’s Great Pyramid of Giza may be hiding a great secret. Using 3-D imaging, scientists recently discovered what appears to be a 100-foot space inside—and it may shed light on how the pyramid was built 4,500 years ago. Ancient Egyptian pharaohs constructed the pyramids as tombs and monuments to themselves, but how they did it is still a mystery. Researchers from Florida’s Heritage Innovation Preservation Institute, who did the 3-D study, say the space may have housed a ramp to carry limestone blocks—weighing up to 15 tons each—to the top of the 455-foot-tall pyramid during construction. They plan to send a tiny drone into the cracks of the pyramid to investigate further. “This pyramid is one of the greatest achievements in the history of architecture,” says Mark Lehner, an archaeologist at Ancient Egypt Research Associates in Massachusetts, “so anything we can learn about its construction is significant.”