Also on board Mars Odyssey is a neutron spectrometer that registers evidence for underground ice in the same regions of the planet.

BBC: Ice reservoirs found on Mars

But Shell says it has a solution for that, a giant underground wall of ice.

NPR: Industry Seeks to Extract Oil from Rocky Mountain Shale

It could identify soil types, verify the presence of water ice and dry ice at the poles, and even track traces of water underground.

NEWYORKER: The Martian Chroniclers

Other ideas include drilling into the aquifer beneath the city and pumping cool water along the tunnels (a method already used to cool the Royal Festival Hall), or installing chillers in the trains that would freeze blocks of ice while the trains were above ground which would then keep them frosty on the underground sections.

ECONOMIST: London Underground