If confirmed, it would catapult Titan into an elite class of solar system moons harboring water, an essential ingredient for life.
Titan boasts methane-filled seas at the poles and a possible lake near the equator. And it's long been speculated that Titan contains a hidden liquid layer, based on mathematical modeling and electric field measurements made by the Huygens spacecraft that landed on the surface in 2005.
The latest evidence is still indirect, but outside scientists said it's probably the best that can be obtained short of sending a spacecraft to drill into the surface -- a costly endeavor that won't happen anytime soon.
The research looks convincing, said Gabriel Tobie of France's University of Nantes.
"If the analysis is correct, this is a very important finding," Tobie said.
The finding by an international team of researchers was released Thursday by the journal Science. The scientists pored over data from the orbiting Cassini spacecraft, which flew by Titan half a dozen times between 2006 and last year and took gravity measurements for a glimpse of its interior.
They found Titan got squeezed and stretched depending on its orbit around Saturn, suggesting the presence of a buried ocean. If Titan were solid rock and ice, such deformations would not occur.
"Titan is quite squishy," noted Jonathan Lunine of Cornell University, who was part of the research team.
Scientists did not delve into the characteristics of the ocean, but previous estimates suggested it could be 30 miles to 62 miles deep and contain traces of ammonia. Titan is one of the few worlds in the solar system with a significant atmosphere, and the presence of an underground ocean could help explain how Titan replenishes methane in its hazy atmosphere.
Having an internal body of water would also make Titan an attractive place to study whether it would be capable of supporting microbial life. Other moons on the shortlist: Jupiter's Europa, where an underground ocean is thought to exist and another Saturn moon, Enceladus, where jets have been seen spewing from the surface.
"Any environment that has liquid water needs to be investigated carefully," said planetary scientist Jean-Luc Margot of the University of California, Los Angeles.
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