On Earth, life has been found deep underground, particularly in the form of bacteria that extract energy from minerals or chemicals created by geologic processes. This has led a number of researchers to hope that similar life may exist on Mars. There is evidence of underground water on Mars, and deep underground the problems of the low temperatures and radiation of the surface would be irrelevant. Also, Mars has a number of (extinct? dormant?) volcanoes, including Olympus Mons, the largest volcano in the solar system.
However, for life to persist on Mars, some kind of ongoing geological activity is probably necessary -- otherwise the minerals and chemicals produced early in the history of Mars would presumably be exhausted by now, and any subsurface bacteria would have starved. This is a big problem; the heavily cratered surface of Mars clearly shows that Mars has had neither significant rain nor robust plate tectonics for billions of years. The standard thinking has been that Mars has never had plate tectonics and that the volcanoes are extinct.
Maybe it's not that bad, though. Images from the Mars Express spacecraft imply that Olympus Mons may have erupted as recently as 2.4 million years ago. That may sound like a long time ago -- and it is, certainly by Earth standards -- but given that the volcano is at least 100 million years old, it would be odd if it died just before we could observe it. It was certainly erupting while sauropods (the long-necked dinosaurs like "Brontosaurus") walked the Earth.
On top of that, there is new evidence that Mars may have limited, much slower, but perhaps still ongoing plate tectonics driving processes similar to some of those on Earth.
In light of all this, wouldn't you expect each Mars lander to contain a seismometer? After all, it is seismometers that have allowed us to determine the existence, depth, and composition of Earth's inner and outer cores; they should be able to provide similar information about Mars, as well as determining more precisely how active Mars is today -- with all the implications that has for the existence of Martian life. As far as I have been able to find, though, there are no seismometers on Mars at all.