A submersible aircraft is the latest addition to DARPA's wish list. In reading through the spec, I can't help but think that perhaps Admiral Nelson helped design it - Admiral Nelson of the fictional submarine Seaview - and that we have a pretty good idea of what it will look like.
The Flying Sub from the Sixties TV series Voyage to the Bottom of the Sea was a genre classic; take a look at the following video, which features a model of the Flying Sub for use in video games.
(Submersible Aircraft video)
DARPA wants more than pretty graphics, though; here is a list of the major requirements.
Flight: The minimal required airborne tactical radius of the sub-plane is 1000 nautical miles (nm). The minimum surface tactical radius is 100 nautical miles. The minimum subsurface tactical range is 12 nautical miles. Note that the ranges quoted are one-way ranges. The platform would need to be able to fly to a location, insert and extract personnel without refueling and this would require the total operational range to be 1000 nm airborne, 200 nm surface, 24 nm under water.
Loiter: The platform should be capable of loitering in a sea-state five, in theater between inserting and extracting personnel for up to 3 days (72 hours). The craft does not need to be submerged during loitering operations; it can operate at the surface.
Payload: The platform should be capable of transporting 8 operators, as well as all of their equipment, with a total cargo weight of 2000 pounds.
Depth: The operating depth of the platform will be constrained by balancing the need to reduce depth in order to minimize structural loads and snorkel complexity with the need to increase depth in order to minimize any potential signatures that could be generated by perturbing the free surface. The effect that the submerged platform will have on the free surface is exponentially proportional to the depth, therefore the platform should be able to operate at a relatively shallow depth and only have the snorkel affect the free surface.
Speed: The speed of the platform in each mode of operation must allow the system to complete a tactical transit (1000 nm airborne,100 nm surface ,12 nm sub-surface) trip in less than 8 hours. This 8 hour time must include any time required by the platform to reconfigure between modes of operation.
These specifications will do - for a start. However, fans of the original TV series know that DARPA has barely scratched the surface of Flying Sub requirements. For example, I don't see anything about "...able to escape the clutches of terrifying kelp-based sea monsters" in there.
(Flying Sub tries to escape clutches of giant sea monster)