Cosmic rays have been running a slow version of this experiment for as long as air has been thick enough to hit. A fast proton from space strikes a molecule high overhead, the collision throws off short-lived fragments, and some of those fragments decay into muons. A muon behaves a lot like an electron, except it carries about 200 times the mass. Extra mass means the particle sheds energy slowly, so it can cross meters of rock or metal that would stop an X-ray in a thin sheet. Archaeologists already use that natural drizzle to hunt hollow rooms inside pyramids, and engineers use it to look for gaps in ground that drills have not reached. Nature only supplies about one muon per square centimeter each second, which is why a careful scan can take months.