Today's Image of Mars comes from Mars Express and shows some pit-chains near the bottom of one of the largest volcanoes in the solar system, Alba Mons. These pits are indicative of lava tubes below the surface, that form in volcanic regions where lava flows solidify near the surface, but continue flowing underneath. When the lava ceases flowing the caverns are left empty and sometimes the roof of the caverns will collapse, leaving the pits you see below. Lava pits like these are prime targets in the search for life on Mars.
One of the reasons scientists believe they have not yet detected any life on the surface is that the radiation levels are 250 times that experienced on the surface of Earth. This would be enough to kill most forms of life as we know it. What makes the lava tubes such a great target is that they are underground and shielded from much of the radiation that hits the surface. They have also been heated by the volcanic activity in the region and have likely experienced period when they were immersed in water, two key factors for life as we know it.
Clicking on this image will take you to the original high resolution image from Mars Express. [See their article on these collapse pits]