Mars Odyssey, which was orbiting around Mars since October 2001, has apparently discovered some caves near a volcano called Arisa Mons. The caves, named Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne - could be entrances to an underground cave system beneath the surface. Scientists are now looking at the possibility that these cave systems could harbor Martian lifeforms.
...and in the future, Martian settlers could look forward to add spelunking to their daily activities in the red planet.
Mars Odyssey website is here.