Earth is the only planet we know of that supports life, but Saturn's moon Titan seems like a potential candidate as well. Many observations made with Cassini and Huygens reveal a host of clues that the moon could be habitable for alien life forms.
Among the discoveries made were lakes and seas of hydrocarbons like methane and ethane, which NASA is eager to explore. To do that, the space agency wants to develop a space submarine that could dive beneath the surfaces of these lakes and seas.
Such a probe would come equipped with cameras, a sonar array, a meteorology sensor, and a retractable sail. These components would allow the probe to present planetary scientists with the answers they're currently foaming at the mouth for.
Of course, there are several challenges to overcome before such a project could materialize, and it would take a while for the probe to get to Titan in the first place. That said, don't hold your breath.