Mars' Moon Phobos has been analyzed as being one-third hollow according to European Space Agency reports, which has triggered some wild and utterly fascinating rumors and speculation that we've featured below.
From "The Phobos Blog" -- published on March 25th: General , Science 25 March, 2010 17:21
Radio science result from 2008 Phobos Flyby now accepted for publication:
We report independent results from two subgroups of the Mars Express Radio Science (MaRS) team who independently analyzed Mars Express (MEX) radio tracking data for the purpose of determining consistently the gravitational attraction of the moon Phobos on the MEX spacecraft, and hence the mass of Phobos. We conclude that the interior of Phobos likely contains large voids. When applied to various hypotheses bearing on the origin of Phobos, these results are inconsistent with the proposition that Phobos is a captured asteroid.
MARSIS was physically seeing (via this radar) a three-dimensional, totally artificial, interior world ... within Phobos; and a "reflection void interior geometry" ... which correlated eerily with the earlier (lower-resolution) Phobos "interior gravity tracking data ...." No natural "space rock" could possibly possess such an enormous range of "natural radar absorbers and reflectors"; nothing "natural" could reflect (or absorb) EM energy that way across so many orders of magnitude.
In other words -- the MARSIS radar reflections officially published on the official ESA Phobos website... contained explicit scientific data, from multiple perspectives, which strongly "supported the idea that this is what radar echoes would look like, coming back from inside 'a huge ... geometric ... hollow spaceship'
In fact, they were the primary source of the decidedly "internal, 3-D geometric-looking" radar signature.
The concurrence of all three of these independent Mars Express experiments -- "imaging" ... "internal mass distribution" (tracking) ... and "internal radar imaging" -- now agreed that "the interior of Phobos is 'partially hollow ... with internal, geometric "voids" inside it ....'" Meaning that- Phobos is artificial.