A new photograph of Mars suggests that huge lakes of liquid water might lie underneath the Red Planet's surface.
The find has raised hopes that life may have once existed on the planet - and might even still be there.
Nasa said that the image "adds to an increasingly complex picture" of our nearest neighbouring planet.
Researchers analysed data from the Mars Reconnaissance Orbiter spacecraft as it looked down on the floor of the McLaughlin Crater, which is 57 miles in diameter and over 1.4 miles deep.
The picture showed that the crater was once deep enough to let underground water flow into its interior.
There is no water there now, but flattened rocks in layers on the crater's bottom, which include carbonate and clay minerals, suggest it was once abundant inside the crater.
It appears that the crater was filled with water from an underground lake, rather than being washed in from outside, Nasa said.
"Taken together, the observations in McLaughlin Crater provide the best evidence for carbonate forming within a lake environment instead of being washed into a crater from outside," said Joseph Michalski, lead author of the paper.
Researchers cited the fact that on Earth half of all living matter is made up of microbes living up to 3 miles underground.
The Mars Orbiter is currently working alongside other craft, including the $2.5 billion Curiosity rover currently drilling rocks on Mars' surface, in an effort to find evidence of life having once existed on the planet.