Here Be Dragons
Click on it for the original. It's a picture of a "subterranean void" on Mars, taken by the HiRISE ultra-high-resolution camera on the Mars Reconnaissance Orbiter. The resolution on that photo is 25 centimeters per pixel; the void shown is about 100 meters across.
This is one of seven holes in Mars; all seven are along the flank of Arsia Mons, the southernmost of the Tharsis volcanos. Presumably they're cave entrances, but -- so far -- even the HiRISE camera can't see anything in there. Mars has a dusty atmosphere; if these were shallow depressions or cave openings, scattered light would be visible in enhanced images. But absolutely nothing is visible. At the very least, that means they're really, really deep.
What's particularly exciting about these caves is that they may be the best places to find extant life on Mars. According to USGS scientists (PDF):
Subterranean void spaces may be the only natural structures on Mars capable of protecting life from a range of significant environmental hazards. With an atmospheric density less than 1% of the Earth’s and practically no magnetic field, the Martian surface is essentially unprotected from micro-meteoroid bombardment, solar flares, UV radiation and high-energy particles from space.
Thermal imaging of the voids show that they maintain a relatively constant temperature, remaining relatively warm in the cold Martian night.
Who's up for a bit of spelunking?