There are few places on Earth where you’d expect to find a neighborhood of crabs, sponges, anemones, and other assorted marine weirdos flourishing atop thousands of barrels of radioactive waste. Three miles beneath the North Atlantic, apparently, is one of them. The French research vessel Pourquoi Pas? recently returned from an expedition to one of the stranger, and more consequential, dump sites on the planet: a vast field of radioactive waste barrels abandoned on the deep-ocean floor by France and other European nations between 1950 and 1990. During 20 dives conducted between May 27 and June 28, a team of roughly 30 scientists descended to depths exceeding 15,400 feet as part of a research project led by Clermont Auvergne University and École Normale Supérieure. Their mission: find out what nearly eight decades of sitting at the bottom of the Atlantic has done to the barrels, the seafloor, and everything living around them. The answer, at least so far, is complicated.…