The interpretation is based on images taken by the High Resolution Stereo Camera on Europe's Mars Express spacecraft. These show extensive fields of large, platy features - reminiscent of the fractured ice floes found in polar regions on Earth.More details at BBC
Huge frozen sea found just below surface of Mars

A team of European scientists from ESA announced the discovery of a huge frozen sea under the surface of Mars over an area of 800 by 900km.