caves, water on mars

The Mars Express Probe is circling Mars, gathering data, and sending the data through space back to Earth. Its radar equipment is known as the MARSIS radar (that stands for "Mars Advanced Radar for Subsurface and Ionospheric Sounding". Data has recently been analyzed from the probe on the Martian polar caps, which appear to be water. This is interesting, because water is believed to be essential for life.
From spaceref.com:

the study of the Marsis radar signal has revealed that the South Polar region of Mars, which has the shape of a giant dome of around 1000 km diameter, is mainly composed of ice.
...it has been possible to estimate the total volume of ice in this region at 1.6 million cubic kilometers. If this volume of ice was distributed in a uniform manner over the whole surface of the planet, Mars would be covered by 11 m of water.

A sea of water eleven metres deep over the whole planet! (if it were evenly spread). That's a lot of water.
Another recent discovery is the existence of underground caves on the surface of Mars.
Or to speak more precisely, there are several large dark spots near the equator that appear to be caves. One of them is estimated to be as much as 130 metres deep. These would make a suitable place
If they are in fact caves, then these would be a place to search for signs of life on Mars. If microbial life on other planets does exist, then it would be more likely to survive -and its fossils to be preserved - somewhere away from the surface bombardment of meteors from outer space.
This is by no means certain, but the answer is interesting either way. As science fiction author Arthur C Clark once said:

"Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case the idea is quite staggering."