For thousands of years, archaeologists and grave robbers have searched every nook and cranny of the Great Pyramid, second-to-last resting place of the ancient Egyptian ruler Khufu — but the ancient tomb has held onto a few secrets.

Today, tourists enter the Great Pyramid of Giza through the Robber’s Tunnel: an entrance dug into the pyramid’s north face by medieval looters on the orders of a ruler named al-Ma’mun, whose army had swept into Egypt from what’s now Iraq. A few yards above the Robber’s Tunnel entrance, another missing chunk of the pyramid’s northern face reveals four enormous limestone blocks arranged in a chevron shape. And behind that stone chevron, inaccessible from outside, lies a hidden corridor.