This force propels the swimmers out of the water, where they then fall forward, generating a horizontal wave.

MSN: Fastest swimmers make webbed hands out of water

This means that normal matter collapses more slowly than dark matter, and that when it does so it has a tendency to oscillate, generating a sound wave.

ECONOMIST: Astronomy

The scheme is part of efforts to develop the Pentland Firth and waters surrounding the Orkney Islands in the largest planned wave and tidal generating programme in the world.

BBC: Pentland Firth tidal project set for development

But how much more could it be when offshore wind, wave and tidal turbines are in place and generating power?

BBC: Balance of payments