Click for original figure from paper.
Using data obtained from Europe’s Mars Express orbiter, scientists believe they have detected evidence of a very large and extensive deposit of buried ice in the dry Martian equatorial regions, buried within the Medusae Fossae Formation, what is thought to be the largest volcanic ash deposit on Mars.
The blue-to-orange areas inside the Medusae on the map to the right, taken from figure 5 of the paper, shows where they have detected potential buried ice, at depths ranging from one to two thousand feet below the surface. The orange areas indicate the thickest ice deposits, as much as two miles thick. From the paper’s abstract:
The MARSIS radar sounder [on Mars Express] detects echoes in Medusae Fossae Formation deposits that occur between the surface and the base which are interpreted as layers within the deposit like those found in Polar Layered Deposits of the North and South Poles. The subsurface reflectors suggest transitions between mixtures of ice-rich and ice-poor dust analogous to the multi-layered, ice-rich polar deposits.
Assuming this detection is real, this would be the largest reservoir of potential water in the dry equatorial regions found yet, comparable to another similar buried detection deep below the giant canyon Valles Marineris but much larger.
Accessing this water however will not be simple, as it is deep underground. You couldn’t just drill a well, as it is ice, not a liquid water table. It would have to mined like minerals on Earth. There are uncertainties about this conclusion as well. It is possible the detection is not water but volcanic ash or dust compacted in a way that mimics an ice detection.