London Telegraph 
July 5, 2010
Pentagon researchers are attempting to develop a military vehicle which can travel underwater like a submarine before bursting out of the waves and flying like an aeroplane.
The Defense Advanced Research Projects Agency (DARPA), the US military science and technology department, has set about creating an aircraft that can fly low over the water until near its target before disappearing under the sea to avoid detection.
It would then creep closer in submarine form before attacking its target, probably a ship or coastal installation, and fly home.
New Scientist reports  that the project, which has been in development since 2008, has reached design proposal stage, and several outside developers have submitted designs. DARPA could start allocating funding to developers in as little as a year.
While the principles of hydrodynamic and aerodynamic flight are similar, the technological challenges are profound. Aircraft need to be as light as possible, so that they can use a minimum of power to get airborne, while submarines need to be dense and strong to withstand water pressure. Heavier-than-air aircraft get their lift from airflow over their wings – submarines simply pump water in and out to change their buoyancy.