In 2011, a curious marine scientist captured a series of photos of the ocean floor that left him disturbed. Using a sea drone, he documented dozens of corroding industrial barrels, scattered 12 miles off the coast of Los Angeles. Tests later showed that the sediment inside contained exceptionally high concentrations of DDT, a pesticide banned in the 1970s, and other chemical waste. A decade later, that scientist, David Valentine, a professor of biology and earth science at the University of California, Santa Barbara, has an answer. They counted more than 25,000 barrels that they believe may contain DDT-laced industrial waste.