Archaeologists have discovered a huge, unidentified space deep inside the Great Pyramid of Giza, the massive ancient structure built on the outskirts of Cairo some 4,500 years ago.

The international team of scientists has been using a sophisticated imaging technique based on cosmic radiation to peer inside the pyramid and map its internal structure. Now, after two years of study, they have identified a mysterious void hovering just above the pyramid’s impressive Grand Gallery—the sloping cathedral-like passageway at the center of the pyramid that links the Queen’s burial chamber to the King’s above it.

Researchers believe the newly discovered space has similar dimensions to the gallery, which measures almost 50 meters (164 feet) long and 8 meters (26 feet) high. But they are not sure whether it’s simply a structural feature designed to relieve weight on the gallery or whether it serves some other—possibly more important—purpose.