[yodon]

A team from Berkeley led by Luis Alvarez used Cosmic Rays to image the inside of the Great Pyramid in the 1960's[0]. tl;dr it's solid rock. My favorite story from the effort was they briefly had incredible excitement because they found a chamber. The excitement lasted until they realized they had neglected to include the room where the detectors were located, which the system detected correctly, in their analytical model of the structure (providing their method was able to identify unpredicted rooms in the structure).

[0]https://www.science.org/doi/abs/10.1126/science.167.3919.832