Over 27,000 barrels of the toxic insecticide DDT have been found so far on the seafloor about 12 miles off the coast of Los Angeles, in what could be one of the greatest examples of industrial pollution uncovered in recent memory.

The barrels have been leaking, and researchers fear that there could be up to a few hundred thousand barrels of DDT waste in total. Over 100,000 total objects have been found in the area by researchers at the University of California San Diego’s Scripps Institution of Oceanography.

The barrels cover an area roughly spanning double the size of Manhattan and lie off the coast of Santa Catalina Island, which is home to dozens of endemic species that exist nowhere else in the world.

DDT waste has been linked to cancer and widespread disease among humans as well as mass die-off events in the natural world. It is likely that the vast trove of illegally dumped DDT could be linked to the widespread cancer faced by sea lions along the West Coast.

“Unfortunately, the basin offshore Los Angeles has been a dumping ground for industrial waste for several decades, beginning in the 1930s. We found an extensive debris field in the wide area survey,” said Eric Terrill, chief scientist of the expedition and director of the Marine Physical Laboratory at Scripps Institution of Oceanography, said in a statement.

Los Angeles Times reports that shipping logs from a disposal company implicate Montrose Chemical Corp. of California, a company that produced DDT, in likely dumping some 2,000 barrels of DDT-laced sludge each month from 1947 to 1961 into a designated dumpsite.

Additionally, logs from other entities show that several other industrial concerns in Southern California used the basin as a dumping ground until 1972, when the Marine Protection, Research and Sanctuaries Act was enacted.