Around 27,000 objects - potentially barrels containing the banned pesticide DDT - have been found dumped off California.
The location, near Catalina Island and 12 miles from LA, has long been suspected of being a toxic waste site stretching back to the Second World War.
High levels of DDT have been detected in marine mammals in the area, and it has also been linked to cancer in sea lions.
Some 27,345 "barrel-like" images were identified by an underwater drone guided by the Scripps Institution of Oceanography at University of California in San Diego.
Researchers used sonar to map more than 36,000 acres between the Los Angeles coast and Catalina Island.
They began where 60 barrels were discovered a decade ago.
The new find showed massed of barrels 900m (3000ft) below the surface and covering a "staggering" area, said head of the mission Eric Terrill.
More than 100,000 "total debris objects" were found on the seafloor, researchers added.
Shipping logs show industrial companies used it as a dumping ground until 1972, when the Ocean Dumping Act was introduced and DDT banned.
An estimated 350-700 tonnes was dumped but it will be up to others to confirm via sampling if the barrels do contain the pesticide.
The long-term impact of DDT on marine life is still not known, said Scripps chemical oceanographer and professor of geosciences Lihini Aluwihare.
But in high doses the substance is considered a possible carcinogen in humans and studies have also suggested "reproductive effects", according to the US Environmental Protection Agency.
The barrels could be moved to a safer location if they haven't leaked, said University of Buffalo chemistry professor Diana Aga.
If they have leaked, samples from sediment, water and marine life could be used to assess the damage.
The survey, which took place from 10-24 March, followed a Los Angeles Times report last year.
The paper exposed shipping logs from Montrose Chemical Corp which showed 2,000 barrels of DDT-laced waste were dumped in the ocean every month from 1947 to 1961 off Catalina, and that other companies dumped there until 1972.