Off the coast of Scotland, in the choppy sound between Caithness and the Orkney Islands, you can find some of the fastest-flowing marine waters in the world. The tides of the Pentland Firth can churn at rates of up to 18 miles per hour. Rough waters also mean energetic waters; the Pentland Firth, some believe, may help to generate enough energy to power a third of Scotland.

The vehicle for that power-provision? Turbines. But not the wind turbines that dot the marine horizon, in the manner of Cape Cod's wind farm; Pentland Firth's turbines will generate their power from the bottom of the ocean.

Underwater windmills, the BBC notes, have the benefit of invisibility—a common objection to wind turbines being how unsightly they are to human eyes. Undersea turbines also benefit from the fact that tides are predictable in ways that winds are not: You know how much power you're generating, basically, on any given day.

For that reason, some are referring to Scotland as "a Saudi Arabia of renewable energy potential."

This fall, the green-tech firm MeyGen will install a turbine farm off the Scottish coast—with the goal to make Scotland "a world leader for turning sea flow into electricity."

MeyGen will face a challenge in that work: The turbines, as you might imagine, are incredibly difficult to install. The Pentland Firth is a harsh environment to begin with; complicating matters is the fact that the turbines can be installed only at the deepest of ocean depths so as not to disrupt the paths of ships on the surface. They also need to be installed in bays or headlands, where tidal flows are at their most intense.