Muon tomography
Muon tomography images the inside of solid things using particles made by cosmic rays. In 2017 it found a void at least 30 m long inside the Great Pyramid, without drilling a hole.
Nobody agrees how Khufu's pyramid was built, and any look inside must leave it undamaged. Cosmic rays hitting the upper atmosphere make muons, which reach the ground at about 10,000 per square metre every minute and can cross hundreds of metres of stone. In the Queen's chamber, inside the 139 m pyramid, only about 1 percent got through, so counting took months. In 2017 the extra muons revealed a void at least 30 m long: the first major find inside since the 1800s, its purpose unknown.
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