Particles raining down from space revealed a hidden chamber inside the Great Pyramid

Cosmic rays constantly shower the Earth, and the muons they create pass through solid stone, losing energy depending on how much rock they cross. In 2017, the ScanPyramids project placed detectors inside and beside the Great Pyramid of Giza and spotted a muon shadow revealing a previously unknown void above the Grand Gallery, roughly the size of an airliner.

— Kunihiro Morishima et al. (ScanPyramids mission), Discovery of a Big Void in Khufu's Pyramid by Observation of Cosmic-Ray Muons — Nature, 2017