The US Department of Defense is launching a research effort to develop underwater robots/sensor platforms that would hibernate on the ocean floor until they "wake up when commanded, and deploy to surface providing operational support and situational awareness." DARPA has dubbed the research effort the Upward Falling Payloads program. (The image below, from the DARPA press release, seems to be illustrating, um, a robot's-eye-view as it's surfacing.) From their announcement:
Depending on the specific payload, systems would provide a range of non-lethal but useful capabilities such as situational awareness, disruption, deception, networking, rescue, or any other mission that benefits from being pre-distributed and hidden. An example class of systems might be small unmanned aerial vehicles (UAVs) that launch to the surface in capsules, take off and provide aerial situational awareness, networking or decoy functions. Waterborne applications are sought as well.