Nasa's Cassini probe has found evidence for seas, probably filled with liquid hydrocarbons, at the high northern latitudes of Saturn's moon Titan.
The dark features, detected by Cassini's radar, are much bigger than any lakes already detected on Titan.
The largest is some 100,000 sq km (39,000 sq miles) - greater in extent than North America's Lake Superior.
It covers a greater fraction of Titan than the proportion of Earth covered by the Black Sea.
The Black Sea is the Earth's largest inland sea and covers about 0.085% of our planet's surface.
The newly observed body on Titan covers at least 0.12% of that world's surface. Cassini team members argue that this gives them reason to call it a sea.
Since Cassini's radar has caught only a portion of each of the new features, only their minimum size is known.
Methane and ethane
Titan is the second largest moon in the Solar System and has a diameter that is almost twice that of Earth's Moon.
While there is no definitive proof that these seas contain liquid, their shape, their dark appearance in radar that indicates smoothness, along with other properties, point to the presence of liquids.
The liquids are probably a combination of methane and ethane, given the conditions on Titan and the abundance of methane and ethane gases and clouds in Titan's atmosphere.
"Carl Sagan [the astronomer] said that Titan must be covered with oceans, and that these replenished the atmosphere with methane," explained Charles Wood, chair of space studies at the University of North Dakota, US.
But that vision of hydrocarbon oceans had to be scrapped when Cassini peered through Titan's organic haze and found a chaotic, geologically active surface, but no large bodies of liquid.
Recently, the orbiter started to spot lake-like features at Titan's northerly regions.
"They are limited to about 65 degrees latitude," said Charles Wood, although he added that river-like channels seemed to be everywhere on Titan.
The discovery of these even larger bodies might hold the solution to the problem of replenishment of the atmosphere with methane.
After making these findings, Cassini team members plan to re-point the orbiter's radar instrument during a May flyby so that it can pass directly over the large dark features imaged by the cameras.
The Cassini-Huygens mission is a cooperative project of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).
Paul.Rincon-INTERNET@bbc.co.uk