Anyone who has been caught in a nasty undercurrent knows the power of the tides. And just as windmills can covert the physical energy in a breeze into electricity, tidal turbines can do the same for the motion in the ocean. The engineering principle is no different — the steady currents found in coastal of water like Canada's Bay of Fundy turn the rotors of an underwater turbine. But tidal streams are far more predictable than wind, which means utilities using tidal don't have to worry about unexpected still days. The cost of putting equipment under water and concerns about the impact on the marine environment have kept tidal experimental, but that's beginning to change. Verdant Power recently completed a successful trial in New York City's East River, and the UK-based Marine Current Turbines is preparing for a 10.5 MW project off the coast of Wales.