With Science and science fiction being close related, Let's talk about Iapetus, or occasionally Japetus, is the third largest satellite of Saturn, eleventh largest in the Solar System, and the largest body in the Solar System known to not be hydrostatic equilibrium.
The orbit of Japetus around Saturn is somewhat unusual - all but one of Saturn's other moons orbits at the same level as the ring system, ie "in the same plane" -Japetus orbits at an angle inclined to the ring system of about 15 degrees. Japetus is not from, nor did it originate with Saturn or the rest of the moon but joined the system at a later time.
By contrast, Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It is thought to be a captured dwarf body from the Kuiper belt that was battered out of hydrostatic equilibrium by repeated impacts. Its orbit is inclined at about 5 degrees in the opposite direction.
Iapetus is rumoured to be constructed because of it baffling, highly geometrical, precisely equatorial - 60,000 high. 1200 km long "Iapetus Wall". Without doubt the most astonishing Iapetus discover is while all Moons over 250 km diameter are spherical - Japetus is highly geometrical, apparently an eroded truncated icosahedron.
It was the theory behind Arthur C Clarke's 2001: a Space Odyssey where an Alien monolith was discovered on its surface which was confirmed when the Voyager space probe discovered that there was indeed a black region within the moon's brighter hemisphere. It is also said to be the inspiration for Star Wars - Death Star.
http://i.imgur.com/8FVSqtC.jpg