This sounds like something straight out of a James Bond movie but no, it's real and it's your government: Those way out engineers at the Defense Advanced Research Projects Agency (DARPA) want to build an aircraft that's as capable of zipping through the sky as it is underwater.

The agency's Submersible Aircraft research project is exploring the possibility of making an aircraft that can maneuver underwater with the goal of revolutionizing the US Department of Defense's ability to, for example, bring warfighters and equipment to coastal locations or enhance rescue operations. DARPA said that the concept being evaluated here is for a submersible aircraft, not a flying submarine. It is expected that the platform will spend the bulk of its time in the air and will only spend short periods of time submerged according to the agency.

According to DARPA: "The difficulty with developing such a craft come from the diametrically opposed requirements that exist for an airplane and a submarine. While the primary goal for airplane designers is to try and minimize weight, a submarine must be extremely heavy in order to submerge underwater. In addition, the flow conditions and the systems designed to control a submarine and an airplane are radically different, due to the order of magnitude difference in the densities of air and water."

There are some major requirements of such a craft, DARPA said, including:

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.

DARPA acknowledges the difficulties in designing such a craft and said that prior attempts to demonstrate a vehicle with the maneuverability of both a submersible and an aircraft have primarily explored approaches that would endow flight capability to platforms that were largely optimized for underwater operation. Unfortunately these prior attempts have been unsuccessful largely because the design requirements for a submersible and an aircraft are diametrically opposed.

Interestingly there was a patent issued in 2007 to Gennady Ploshkin, for a disc-shaped aircraft that could take off like a helicopter and submerge like a sub. There have been other sub-plane designs as well. Probably the most famous flying sub was in the TV show Voyage to the Bottom of the Sea.