A pair of sleek, winged machines are “flying” – or at least swimming – beneath the dark waters of the Faroe Islands in the North Atlantic.

Known as “sea dragons” or “tidal kites”, they look like aircraft, but these are in fact high-tech tidal turbines, generating electricity from the power of the ocean.

The two kites – with a five-metre (16ft) wingspan – move underwater in a figure-of-eight pattern, absorbing energy from the running tide. They are tethered to the fjord seabed by 40-metre metal cables.

Their movement is generated by the lift exerted by the water flow – just as a plane flies by the force of air flowing over its wings.

Other forms of tidal power use technology similar to terrestrial wind turbines but the kites are something different.

The moving “flight path” allows the kite to sweep a larger area at a speed several times greater than that of the underwater current. This, in turn, enables the machines to amplify the amount of energy generated by the water alone.

An on-board computer steers the kite into the prevailing current, then idles it at slack tide, maintaining a constant depth in the water column. If there were several kites working at once, the machines would be spaced far enough apart to avoid collisions.