...Specifically, the team used three different techniques for collecting muons, cosmic rays produced naturally in Earth's atmosphere that can penetrate thick rock. Muography works a lot like x-ray photography. A chemically treated film captures the pattern of muons as they pass through the rock, giving us a picture of what's inside. When muons hit solid objects like bricks, their paths are slightly diverted. But when they pass through empty space, like the "void" that the researchers discovered, their paths remain straight.
To get a muograph of the pyramid, the researchers collected muon traces using nuclear emulsion films and scintillator hodoscopes set up in the queen's chamber at the base of the pyramid; they also used gas detectors outside. Next, they used specialized software to analyze the trajectories of the muon trails. After intensive study and comparing results from the three different muon-gathering tests, the team confirmed that there is a void space—perhaps a chamber or corridor—above the Grand Gallery. It appears to be about the size of the Grand Gallery, too, roughly 30 meters long, and is located about 15 to 17 meters above the surface of the ground...