An unprecedented expedition to map parts of Southern California’s deep-sea has uncovered World War II military weaponry littering the seafloor in massive dumping sites off the coast of Los Angeles.

The research effort spearheaded by UC San Diego’s Scripps Institution of Oceanography sheds new light on exactly what objects lay beneath the surface after thousands of “barrel-like” objects likely containing toxic chemicals were discovered by sonar in stretches of the seafloor in 2021.

Made public on Friday, the latest survey using sonar and robotic vehicles found a multitude of discarded munitions boxes and other explosives along debris lines between Los Angeles and Catalina, thousands of feet below the surface where they were previously identified.

Among the items discovered were depth charges typically used to attack submarines, Hedgehog and Mark 9 depth charges, and Mark 1 smoke floats, which are chemical smoke munitions that were often used by warships to conceal their movements.

Researchers say that these were likely discarded by Navy warships headed back to port — a practice that military officials confirmed was common in years past.

In a statement to the Scripps team, U.S. Navy officials said that “disposal of munitions at sea at this location was approved at that time to ensure safe disposal when naval vessels returned to U.S. port.” They added that officials are reviewing the findings to determine “the best path forward to ensure that the risk to human health and the environment is managed appropriately.”

These findings, scientists explained, are an important milestone in the exhaustive efforts to better understand the impacts of industrial dumping into the waters off the Southern California coast between the 1930s and 1970s, leading to a presence of the pesticide DDT that has plagued marine life in the area.

“This is a really interesting piece of work, because what it demonstrated or what it validated is that in fact there aren’t a huge, vast quantity of barrels down there — at least not what we were thinking,” Brice Simmons, a marine biologist with Scripps, said during a press briefing Friday. “It turns out that a lot of the dumping was simply bulk material dumped off to the side.”

As the researchers explained, the barrel-like objects found in two of the dumpsites identified in earlier surveys using primarily sonar technology had near identical dimensions to the WWII munitions. Even more advanced sonar technology and use of a high-definition deep-sea camera helped researchers distinguish between the two.

However, researchers say it will still take time to understand how massive amounts of DDT ended up in these areas, as well as how far it may be spreading through the food chain from the seafloor.

“There clearly is DDT present. One of the challenges working in the deep sea is being able to survey large areas at high resolution,” said Eric Terrill, the director of Scripps’ Marine Physical Laboratory. Terrill co-led the latest deep-ocean survey with researcher Sophia Merrifield.

“There is a lot of cloudiness around how the DDT actually arrived at the dump site: was it in the form of containerized waste or was it bulk dumping?” he continued. “What this does right now is set the stage for understanding an improved strategy about how to sample and understand the impacts to the environment.”

Preliminary analyses suggest that it may have been through less visible pollution like the bulk dumping of DDT directly into the environment rather than through the discarding of containers.

“Our analysis of sediments are showing that bulk dumping of DDT acid waste was the norm, that DDT immediately entered the environment and was likely not in barrels,” said David Valentine, a researcher with the UC Santa Barbara team who first came across dozens of barrels — the contents of which remain a mystery.

“Once dumped, DDT spread at the seafloor, expanding its footprint to at least the base of the Catalina slope,” he continued. “We are finding that original DDT remains abundant in the seafloor today, in both absolute and relative terms.” 

According to Scripps, there remains a plethora of information that its research team has yet to piece together — findings that can help inform additional studies of DDT and seafloor exploration at large.

“Our survey provides an opportunity to develop and apply analytical techniques to acoustic and optical imagery over wide-areas,” said Merrifield, an observational physical oceanographer who specializes in ocean robotics. “We anticipate these datasets will inform additional studies addressing impacts of dumping activities on the marine food web.”