Cosmic rays were used to probe the Pyramid of Khafre

Between 1967 and 1968, the physicist Luis Walter Alvarez searched for secret rooms in the Pyramid of Khafre using a completely novel method. This mission called Giza Project 1968 consisted of measuring the energy of cosmic ray particles called muons to map the interior of the monument. The experiment was a failure, because the Nobel Prize winner was unaware at the time that the pyramid contained granite denser than the limestone.