It is probably inevitable that man will at some point travel to Mars, and chances are it will be a company behind the expedition rather than NASA but at least now we know that they'll be able to do some cave spelunking thanks to a discover by NASA's Mars Orbiter which is currently sending incredibly high-rez images back to earth.
One of the most recent and amazing discoveries made by NASA scientists who are pouring over those images is what looks very much like a large cave at the bottom of a large bowl shaped depression on the surface of Mars.
Earlier this year, the CTX camera team saw a crater containing a dark spot on the dusty slopes of the Pavonis Mons volcano. We took a closer look at this feature with HiRISE and found this unusual geologic feature.There are plans for the HiRISE team to acquire a second image of the location and then create a stereo pair which they hope will let them unravel this mystery a little further.
The dark spot turned out to be a "skylight," an opening to an underground cavern, that is 35 meters (115 feet) across. Caves often form in volcanic regions like this when lava flows solidify on top, but keep flowing underneath their solid crust. These, now underground, rivers of lava can then drain away leaving the tube they flowed through empty. We can use the shadow cast on the floor of the pit to calculate that it is about 20 meters (65 feet) deep.