I last reported on this two years ago: 2012.antville.org

According to a doco I just saw on TV, it will be another two years before they have detected enough muons to have enough data to know the answer.

The largest particle detector in Mexico is being built inside a pyramid in the ancient settlement of Teotihuacan. The equipment will detect muons, tiny particles that are created when cosmic rays bombard the Earth's atmosphere. Dr Arturo Menchaca and colleagues from Mexico's National Autonomous University hope that by tracking the muons through the pyramid, they can find cavities. The experiment is costing half a million dollars. At the moment, it resembles a large, flat metal plate, connected to a box of wires with a monitor displaying a flickering yellow line. This is the machine that tracks the muons, sub-atomic debris left over when cosmic rays smash into molecules in the Earth's atmosphere. They travel at near the speed of light and pass through solid objects, leaving tiny traces. When a muon hits the receptor, the yellow line leaps up and down in spikes. "The idea is to try to discover density variations in the pyramid," Dr Menchaca told BBC News Online. "These cosmic rays are very penetrating radiation. Some of them go through this pyramid, and some of them are absorbed. "The amount which is absorbed depends on the material which it finds. If we find more muons than we expect, then there is less matter in that part of the pyramid". Less matter could indicate the cavity of a burial chamber.

The clue to why the pyramids exist is that some muons make it through the pyramid, some don't. Outside without protection, they'll all hit you. In a cosmic ray storm you would be safer under the pyramid.

Source: BBC