It’s an ill-kept secret that Russian penetrating radar has shown rectangular interior spaces.
Phobos and Deimos are small, and are obviously either captured asteroids, or were formed from ejecta of impacts on Mars. Phobos has been speculated to be 25-30 percent empty space, iow, a pile of aggregated debris, basically a pile of rubble based on a one or a few largest pieces — much like every other asteroid that has been studied from close up.
The radar study of Phobos was MARSIS, and was by the ESA,, not the Russians.
There was a Russian (alleged) scientist who started claiming back in the 1950s that Phobos was moving irregularly. Naturally, at that time the mass of Mars wasn’t accurately known, and neither was the mass of Phobos for that matter. Here’s fringe website Rense’ page on this:
http://rense.com/general20/eisenhowerWH.htm
Better:
http://www.universetoday.com/58923/could-phobos-be-hollow/
And oddly enough, Wikipedia has the inside scoop:
http://en.wikipedia.org/wiki/Moons_of_Mars#Mars_moon_hoax
Martian moons: Phobos -- The team concluded that Phobos is likely to contain large voids, which makes it less likely to be a captured asteroid. Its composition and structural strength seem to be inconsistent with the capture scenario. It is possible that Phobos formed in situ at Mars, from ejecta from impacts on the Martian surface, or from the remnants of a previous moon which had formed from the Martian accretion disc and subsequently collided with a body from the asteroid belt. Data from the Mars Express OMEGA spectrometer suggests Phobos has a primitive composition, so primitive materials must have been available for accretion during its formation. The circular orbit suggests that Phobos formed in situ whilst analysis of the Planetary Fourier Spectrometer data from Mars Express also points towards in situ formation but does not rule out the possibility that Phobos is a captured achondrite-like meteor.