"Scarring the southern highlands of Mars is one of the Solar System's largest impact basins: Hellas, with a diameter of 2300 km and a depth of over 7 km.....the atmospheric pressure is about 89% higher than at the surface, which may even offer conditions suitable for water."
http://phys.org/news/2014-08-mars-deep.html
Seem interesting to start a colony at that place if there are a lot of water of course.