Similar underground oceans have been found on Enceladus, Saturn's sixth-largest moon, and on Europa, Jupiter's sixth moon.

Scientists said the water suggests the possibility of life.

"This is a scientific result of the first order," said Enrico Flamini, head of space exploration for the agency. "If there is water on Titan, it is important to study it to see if there are the basic conditions for the formation of organic molecules."

The Cassini-Huygens probe, jointly built by NASA, the Italian agency and the European Space Agency, is the first man-made device to approach Titan and the first to orbit Saturn.

Flamini said the probe measured variations in Titan's movement which scientists believe could only be caused by an underground mass of water.