South Polar Basin

Caption NASA:"An enigmatic large Basin appears in the South Polar Region of Saturn's moon Titan at the center of this Titan Radar Mapper image from NASA's Cassini Spacecraft acquired on June 22, 2009.

The Basin has several possible origins.
It may be a Volcanic Caldera, produced by collapse after cryovolcanic eruptions.
It might be a Modified Impact Basin, partially infilled with sedimentary material.
Alternately, the basin may have formed by some other collapse process related to the presence of Subsurface Methane.

Evidence for lakes elsewhere on Titan suggests that it might have been later partially filled with liquid Methane and Ethane".