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APOD: 2007 May 28 - A Hole in Mars
From NASA:

Explanation: Black spots have been discovered on Mars that are so dark that nothing inside can be seen. Quite possibly, the spots are entrances to deep underground caves capable of protecting Martian life, were it to exist...

The diameter of the hole is about the length of a football field. And we've found seven of these holes so far. The Martians must live inside Mars. We've all seen too many sci-fi movies not to think "spaceport."

More (Windows onto the abyss: cave skylights on Mars) at The Planetary Society:

At its highest resolution of 25 centimeters per pixel, the HiRISE camera can see the detailed shape of the slightly scalloped edge of a hole on the flank of Mars' Arsia Mons (left), but no amount of image enhancement (right) can bring out any further details inside the hole. That means that the walls of the cave are overhanging -- the cave is larger below the ground than the entrance we can see at the surface -- and that it is very deep. Mars' dusty atmosphere produces enough scattered light that "skylight" would illuminate the floor of a shallow cavern well enough for HiRISE to detect it.

The hope for the HiRISE images was that we could see some details from inside the hole. But as you can see by the highly stretched version, there is absolutely nothing visible inside that hole. It's black black black black black. HiRISE is a very sensitive instrument, and Mars' dusty atmosphere scatters quite a bit of light around, so there is certainly light entering that cave hole and bouncing around the interior. But it seems that the cave is so big and so deep that almost none of the light that enters the cave comes out. It's deep, and it's big; the hole that we see really is just a skylight on a big subterranean room. How big? We'll never know for sure without visiting it, but I expect that Cushing and his coauthors and the HiRISE team will be crunching the numbers on the illumination conditions and the sensitivity of the camera to put a lower limit on how deep that cave must be for HiRISE to be able to see nothing at all inside it...

The Society's astronomy blogger is Emily Lakdawalla, who has a master’s in planetary geology from Brown University.

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