Ancient mega-tsunamis on Mars may have left evidence of salty oceans capable of supporting life.
The giant waves, reaching as high as 120 metres (394ft), were caused by a pair of meteor impacts several million years apart, scientists believe.
They scarred the Martian landscape and piled sedimentary deposits along the edge of the planet's northern plains.
The sediments are thought to mark out the shorelines of large expanses of water that once covered the Martian lowlands.
Dr Alexis Rodriguez, from the Planetary Science Institute in Tucson, US, said: " For more than a quarter century, failure to identify shoreline features consistently distributed along a constant elevation has been regarded as inconsistent with the hypothesis that a vast ocean existed on Mars approximately 3.4 billion years ago.
"Our discovery offers a simple solution to this problem; widespread tsunami deposits distributed within a wide range of elevations likely characterise the shorelines of early Martian oceans."
Between the two impacts, Mars went through a period of frigid climate change with liquid water turning to ice.
The second tsunami formed rounded lobes of ice whose structure suggested that the ancient ocean was briny.
Co-author Dr Alberto Fairen, from the Centre of Astrobiology in Madrid, Spain, said: "It is difficult to imagine Californian beaches on ancient Mars, but try to picture the Great Lakes on a particularly cold and long winter, and that could be a more accurate image of water-forming seas and oceans on ancient Mars."
The icy tsunami lobes would be "very good candidates" for places to search for biosignatures of microbial life, he said.
The research is published in the journal Scientific Reports.