Phobos has always been a bit mysterious, but the website for the European Space Agency's Mars Express mission comes right out and says what a lot of people have been thinking for a long, long time:
Something is not right about Phobos. It looks like a solid object but previous flybys have shown that it is not dense enough to be solid all the way through.
The same people who scoff at the idea of a Hollow Earth, or even small hollow pockets deep in the Earth's mantle, are now faced with possibly being forced to accept a body in the solar system that is largely hollow.
The ESA article again:
“Phobos is probably a second-generation Solar System object,” says Martin Pätzold, Universitat Koln, Cologne, Germany, and Principal Investigator of the Mars Radio Science (MaRS) experiment. Second generation means that it coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as the Red Planet. There are other moons around other planets where this is thought to have been the case too, such as Amalthea around Jupiter.
Some researchers think Phobos might be riddled with vast caverns; the Mars Express spacecraft has been making a series of close flybys of Phobos this month, and high-resolution photos could be available any day now.
Phobos, which figures in numerous science fiction stories and real-life conspiracy theories as possibly being the site of an inhabited base (alien, CIA, Soviet, or Nazi, depending on which conspiracy theory you read) has a number of peculiar features that set it apart from other solar system bodies: