NASA Wants to Send a Submarine to Titan

Titan, the largest moon of Saturn, has liquid seas made of methane and ethane. Since it is the only planetary body other than the Earth within our solar system known to have liquid seas, it fascinates scientists. Some want to learn more about those seas in ways that space probes flying by can't. They have a radical proposal to do so: build a submarine and send it to Titan.
Their plan is to build an autonomous submersible probe than can spend 3 months on Titan examining the largest sea there. The proposal offers serious engineering challenges due to the powerful currents and extreme cold on Titan. Discovery News reports:

Envisaged as a possible mission to Titan’s largest sea, Kracken Mare, the autonomous submersible would be designed to make a 90 day, 2,000 kilometer (1,250 mile) voyage exploring the depths of this vast and very alien marine environment. As it would spend long periods under the methane sea’s surface, it would have to be powered by a radioisotope generator; a source that converts the heat produced by radioactive pellets into electricity, much like missions that are currently exploring space, like Cassini and Mars rover Curiosity.
Communicating with Earth would not be possible when the vehicle is submerged, so it would need to make regular ascents to the surface to transmit science data.
But Kracken Mare is not a tranquil lake fit for gentle sailing — it is known to have choppy waves and there is evidence of tides, all contributing to the challenge. Many of the engineering challenges have already been encountered when designing terrestrial submarines — robotic and crewed — but as these seas will be extremely cold (estimated to be close to the freezing point of methane, 90 Kelvin or -298 degrees Fahrenheit), a special piston-driven propulsion system will need to be developed and a nitrogen will be needed as ballast, for example.