Catastrophic melting and outflow of a buried ice lake formed the lumpy, bumpy floor of an ancient impact carter on Mars, an Italian scientist says.

Manuel Roda, a structural geologist at the University of Utrecht in the Netherlands, said such chaotic terrains -- enigmatic features stretching hundreds of miles -- are distinctive to Mars but the mechanism of their formation has been poorly understood.

The so-called Aram Chaos sits within a crater 175 miles wide and more than 2 miles deep.

"About 3.5 billion years ago, the pristine Aram impact crater was partly filled with water ice that was buried under a two-kilometer (1.2-mile) thick layer of sediment," Rhoda said at the European Planetary Science Congress in London.

"This layer isolated the ice from surface temperatures, but it gradually melted over a period of millions of years due to the heat released by the planet. The sediment overlying fluid water became unstable and collapsed," he said.

The resulting massive expulsion of thousands of cubic miles of liquid water -- four times the volume of Lake Baikal, the largest freshwater lake on Earth -- carved a valley more than 6 miles wide and a mile deep in about one month and left the chaotic pattern of blocks in the Aram crater, Rhoda said.

"An exciting consequence is that rock-ice units are possibly still present in the subsurface," he said. "Buried ice lakes testify of Mars rapidly turning into a cold, frozen planet, but with lakes buried in the subsurface. These lakes could provide a potentially favorable site for life, shielded from hazardous UV radiation at the surface."