NEW YORK, NY.- When marine scientists sent an underwater camera down to the seafloor off the Southern California coast, they expected to find the remnants of barrels used to dump chemical waste decades ago.

Instead, they found hundreds of World War II-era munitions, including anti-submarine explosives and smoke devices, dotted along the ocean floor in rough lines stretching over 1 mile.

Eric Terrill and Sophia Merrifield, scientists from the University of California San Diego Scripps Institution of Oceanography, announced Friday the findings of their underwater surveying project, which was conducted in April last year.

The seafloor south of Los Angeles was an industrial dumpsite from the 1930s to 1970s. Previous studies had shown tens of thousands of barrel-like objects littering it, which the scientists believed could contain the chemical waste, including banned pesticide DDT.

“It was a big surprise for us,” Terrill said in an interview. “Our expectations were we’d find fragments of barrels along all these debris lines, and we started coming upon objects that weren’t barrels.

“It’s concerning to not know what’s going on on the seafloor near coastlines,” Merrifield said.

Over three weeks, the researchers used sonar to map a 135-square-mile stretch of the ocean floor between Catalina Island and Long Beach and took 300 hours of high-definition video of the ocean floor. Through a partnership with the U.S. Navy Supervisor of Salvage, they used military technology normally used to find downed planes to map the ocean floor at high scale and clarity.

They found boxes of munitions and Hedgehog and Mark 9 depth charges, which were carried on warships and dropped into the ocean to deter submarines. They also found Mark 1 smoke floats, a type of pyrotechnic used to disguise ships while in combat.

Some of the munitions were similar in shape and size as barrels that have been used elsewhere to dispose of industrial waste. It was only by using video imaging, with the help of an expert specializing in underwater archaeology, that the scientists were able to confirm the objects as munitions.

Terrill and Merrifield said they believed the munitions were linked to the Long Beach Naval Shipyard, where warships were maintained and repaired during World War II and the Korean War, before being shut down in the 1990s.

The U.S. Navy confirmed in a statement that the munitions were “likely a result of World War II-era disposal practices” and that the dumping of munitions at that location were approved at the time to “ensure safe disposal when naval vessels returned to U.S. ports.”

The Navy would be “reviewing the findings and determining the best path forward to ensure that the risk to human health and the environment is managed appropriately,” the statement added.

Recent media attention has renewed public concern about the effect of past industrial dumping off the Southern Californian shore. Adult sea lions in the area have a high rate of cancer, which scientists have linked to the presence of DDT.

DDT, or dichloro-diphenyl-trichloroethane, was used as a pesticide starting in the 1940s. But it was banned in 1972, after mounting evidence of its damage to ecosystems.

The seafloor off the California coast was long known as a dumping site for DDT waste products and other types of industrial waste from the 1930s to 1970s. Scientists are still evaluating the scale of the dumping and its ongoing effects on marine ecosystems.

This article originally appeared in The New York Times.