Just 10 miles off the coast of Los Angeles lurks an environmental disaster over 70 years in the making, which few have ever heard about. That is, until now, thanks to the research of a University of California marine scientist named David Valentine. 

Working with little more than rumors and a hunch, curiosity guided him 3,000 feet below the ocean's surface. A few hours of research time and an autonomous robotic submersible unearthed what had been hidden since the 1940s: countless barrels of toxic waste, laced with DDT, littering the ocean floor in between Long Beach and Catalina Island. 

The fact that his underwater camera spotted dozens of decaying barrels immediately in what is otherwise a barren, desert-like sea floor, Valentine says, is evidence that the number of barrels is likely immense. Although the exact number is still unknown, a historical account estimates it may be as many as a half a million.

After 70-plus years of inaction, Valentine's research has finally helped initiate a huge research effort to reveal the extent of the contamination.

But this offshore dump site is only a part of the story of environmental damage from years of DDT discharge along the coast of Southern California — a story which likely won't be closed for decades to come because of its ongoing impact, including a recently discovered alarming and unprecedented rate of cancer in the state's sea lion population, with 1 in every 4 adult sea lions plagued with the disease.

The chemical DDT was invented in 1939 and used during World War II as a pesticide helping to protect troops from insect-borne diseases like Malaria. After the war, production of the chemical ramped up and it became routinely used in the spraying of crops, and even over crowded beaches, to eliminate pests like mosquitos.

But in the 1960s, DDT was discovered to be toxic. Over time, eating food laced with DDT builds up inside the tissues of animals and even humans, resulting in harmful side effects. The EPA now calls it a "probable human carcinogen." In 1972, when the U.S. government started taking environmental pollution seriously with legislation like the Clean Air Act, DDT was banned in the United States.