NASA announced today that data from its Cassini spacecraft and Deep Space Network shows Saturn's moon Enceladus hosts a fairly large liquid-water ocean beneath its surface, meaning it is now on NASA's short list of planets that could possibly harbor alien life. 

Such a body of water was first hypothesized in 2005, when Cassini spotted vapor and ice coming out of vents near the moon's south pole. The new data, however, offers much stronger evidence of the underground sea, per NASA, which explained in a press release that "the new data provide the first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean inside the moon." 

Sami Asmar, a co-author of the forthcoming article on the findings, explains how his team made the discovery

The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun... as the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we're trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system.

Lead author Luciano Iess offered some more detail

The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the onboard camera... Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir.

Nasa says that the gravity measurements suggest the ocean is likely 6 miles deep, and lies beneath a 19-25-mile-thick icy crust. 

The discovery appears to have struck a nerve: 

Full findings will be published in Friday's edition of Science.