Since launching in November 2011, NASA’s Mars Science Laboratory (MSL) has been on a 560 million-kilometer (350 million-mile) journey to the Red Planet, with landing scheduled for late Sunday August 5 or early Monday August 6, depending on where you live on Earth.
This is NASA’s fourth rover mission to Mars, and its goal is to determine the planet’s past — and present — potential for habitability.
The running theory is that there is nothing presently alive on Mars.
So while the poles have (frozen) water...well, we "already know" they have (frozen) water. Going to all the trouble of landing a probe there to go "yup, that's water" wouldn't really get us very much - we don't expect to find anything alive anywhere we go on the planet. So the thing to look for is where there might have been evidence that life ever was.
And things like a gently rising mountain with exposed sediment layers all the way up its side would offer a rather uniquely good perspective on the history of the surface of Mars. So, given one probe, that's where we went.
This video talks a little bit about the various landing sites that were being discussed (about 4:50 in):
It'll have a rough landing and the chute doesn't detach properly upon landing and drags it hundreds of yards. Many tense hours then await as they try to communicate and run tests after which they find the thing works perfectly...
...except that the systems got jangled enough and in overcoming some of its problems, it developed sentience. A psychologist has to be called in to talk with it because it's rather alone.