Mars seems to have a frozen lake on its surface, according to images obtained by the European Space Agency's Mars Express satellite. One hypothesis has it that Mars sporadically belches up volcanic gases and floodwater, leaving behind huge seas, which then evaporate. Researchers have identified the apparent remnants of such an outburst: a frozen body of water the size of Earth's North Sea. Although some experts dispute the finding, believing it to be lava flow, the formation is fragmented like ice on terrestrial seas. Crater counts indicate the object is roughly five million years old—young in geologic terms—and its horizontal surface, along with several craters that seem partially filled up, suggests that the ice remains to this day. The researchers posit that an eruption from two crevasses known as Cerberus Fossae provided the water, and a layer of volcanic ash settled on the resulting glacier, preventing its sublimation into vapor, they explain in the March 17 Nature.