A dark, lake-like feature on Saturn's largest moon, Titan, has been imaged by the US-European Cassini spacecraft.
Researchers have long speculated that Titan might harbour open bodies of liquid methane - and the 235km by 75km target is the best candidate to date.
But they are being cautious about interpreting the feature, which has what looks to be a smooth shoreline.
The scientists say the object could simply be dark, solid deposits caught in a sinkhole of volcanic caldera.
"This feature is unique in our exploration of Titan so far," said Dr Elizabeth Turtle, Cassini imaging team member at the University of Arizona, US.
"Its perimeter is intriguingly reminiscent of the shorelines of lakes on Earth that are smoothed by water erosion and deposition," she added.
The feature lies in Titan's cloudiest region, towards the south pole, which is presumably the most likely site of recent methane rainfall.
This, coupled with the shore-like smoothness of the feature's perimeter makes it hard for scientists to resist speculation about what might be filling the lake, if it indeed is one.
"An alternate explanation is that this feature was once a lake, but has since dried up, leaving behind dark deposits," Dr Turtle said.
Despite earlier predictions, no definitive evidence for open bodies of liquid has been found on Titan. Cassini has not yet been in a favourable position for using its cameras to check for glints from possible surface liquids in the south polar region.
Thirty-nine more Titan flybys are planned for Cassini's prime mission.
Huygens captured more than 300 images on its descent
In future flybys, the science teams will search for opportunities to observe the lake feature again and to look for mirror-like reflections from smooth surfaces elsewhere on Titan.
Such reflections would strongly support the presence of liquids.
Cassini's Titan probe, Huygens, sent back images on its descent to the surface of the moon of channels and gullies that looked very similar to the liquid-cut features seen on Earth.
The Cassini-Huygens mission is a cooperative venture of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency.