[opendoor]

Originally Posted by GGG

When Cameron’s team designed his sub he had a failsafe to corrode off the ballast and self return to surface in the event of failure of various systems.

http://www.deepseachallenge.com/the-sub/safety/

This one was supposed to have something similar with self-dissolving bonds for the ballast bags. It also apparently had ballast that could be released hydraulically, so they'd still be able to release it in the event of a loss of power. And then in the event of loss of hydraulic power, there was the "lean to one side" mechanical release as well. Who knows which (if any) of those were fully functional though.