Just off the coast of Saint Jean De Luz, near the breakwater that protects the bay from Atlantic swells, floats a cheerful yellow buoy unnoticed by most tourists and beach-goers at the popular resort town across the harbor. It's easy to miss, but it's the only above-water sign of something remarkable on the ocean floor about 15 meters below.

Egiategia (the name is a Basque play on words that roughly translates as "true warehouse or workshop") has been fermenting wine underwater since 2012 using the constant motion of the waves to continuously agitate their wine as it ferments in specially designed vats anchored to the sea floor.

The winery sources grapes from local vineyards, pours the juice into their patented containers, loads them onto a boat, and drops them into the sea by the yellow buoy, where a team of divers secure them to anchors at their "Winereef." They'll remain underwater for periods of six months to a year.

While wine itself is about as dense as water, the yeast responsible for creating…