Scientists have found a hidden chamber inside the Great Pyramid of Giza, a research paper published in the journal Nature on Thursday showed. This is the “first major inner structure” found inside the pyramid since the 19th century.

The 30-metre-long void is above the Grand Gallery of the pyramid, and was found using cosmic ray imaging, through subatomic particles called muons that can penetrate rocks. The Great Pyramid is the largest of the three pyramids of Egypt, and was built during the reign of Pharaoh Khufu between 2509 BC and 2483 BC.

The purpose of the chamber is unclear, and it does not appear to be connected to any known passages inside the pyramid.

A team of Japanese and French scientists made the discovery after two years of study, BBC reported. The chamber was named “ScanPyramids Big Void”.