This weekend, I stumbled across this amazing photo in the German newspaper FAZ, under the headline A "Black Hole" on Mars.
(HiRISE Image PSP_003647_1745, NASA)
What could make easily a perfect belated April Fools's joke is, however, a real photograph! It was taken with the High Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter spacecraft on May 7, 2007, at -5.5 degrees latitude, 118.6 degrees Western longitude on Mars.
Well, on the HiRISE web page it is described innocuously as "Candidate Cavern Entrance Northeast of Arsia Mons" - and that's what it is supposed to be: a round hole with a diameter of about 100 meter in the ceiling of a wide cavern in the Martian ground. It's so deep that the sun, at 03:27 PM local Martian time at about 38 degrees above the horizon, doesn't reach the ground. You should have a look at this detailed photo...
More explanations are given at The Planetary Society Weblog.
So that's where all the Little Green Men are hiding ;-)
Similar geological features (even with about the same diameter) exist also on Earth - for example the Zacaton sinkholes in Mexico. On Earth, these caverns are filled with water (see here for a detailed description as PDF file). The satellite images from the hole on Mars are now better than those of Zacaton at google.maps - that's crazy...
UPDATE (September 7, 2007): New photos, also taken by the HiRISE collaboration, show the shadow of the rim cast onto the wall of the pit. The photo allows to calculate that the pit is at least 78 meters (255 feet) deep, and the new data suggest that the hole has volcanic origins.
TAG: Mars, Zacaton