Aged, hard cheese

Edward Schreiber and Orson Anderson once tested whether the Moon really could be made of green cheese...Schreiber and Anderson compared the speeds of sound waves in rocks that were returned from the Moon with measured sound speeds of various terrestrial materials, including various types of cheese. (Sound speeds correlate highly with density and are thus often used to try to infer the composition of rocks.)

The original paper (a copy, or a PubMed link if you really want it) includes measurements for a variety of Earth-bound minerals, moon rock samples, and seven cheeses, including the Swiss sapsago (a hard cow's milk cheese which is, due to added herbs, a bit green).

There's a lesson here: don't expect the newest hypothesis to explain everything, but don't blindly trust the old hypothesis, either.