SAN DIEGO—A second sweeping survey of the seafloor off the Southern California coast by researchers from the University of California–San Diego’s (UCSD) Scripps Institution of Oceanography revealed thousands of aging military munitions and explosives littering the bottom of the ocean.

The survey, conducted in April 2023, was a follow-up to one completed in 2021, which found thousands of barrel-sized objects organized in lines across 135 square miles of the San Pedro basin—between Santa Catalina Island and Long Beach. The expedition took place with support from the U.S. Navy’s Supervisor of Salvage and the Office of Naval Research.

The researchers wanted to identify the barrel-like objects in the basin, a known location for industrial dumping between the 1930s and 1970s—including dumping of byproducts from the manufacturing of the pesticide DDT.

Example munitions surveyed by a remotely operated vehicle during a seafloor survey, led by the University of California–San Diego Scripps Institution of Oceanography, of the San Pedro Basin in 2023. (Courtesy of University of California–San Diego Scripps Institution of Oceanography)

While industrial waste is indeed confirmed to be on the seafloor, a surprising number of the objects identified were munitions now nearly 80 years old.

“These munitions are likely a result of World War II-era disposal practices,” a statement from the Navy reads. “While disposal of munitions at sea at this location was approved at that time to ensure safe disposal when naval vessels returned to U.S. ports, the Navy follows [the U.S. Department of Defense’s] guidance for the appropriate disposal of munitions that aligns with state and federal rules and regulations.

“The Navy is reviewing the findings and determining the best path forward to ensure that the risk to human health and the environment is managed appropriately and within applicable federal and state laws and regulations,” a statement from UCSD read.

Led by Scripps oceanographers Sophia Merrifield and Eric Terrill, the 2023 survey used a deep-water autonomous underwater vehicle with sonar and a remotely operated vehicle with a high-definition video camera, both capable of working up to full ocean depth of 6,000 meters or 19,600 feet, the statement read.

During the 2023 survey, barrels from the legacy of industrial dumping and several old fishing vessels were also found on the seafloor. The barrels of industrial waste were found to be concentrated in two locations, and barrels were not pervasive across the dump site, the researchers said.

“The resolution of the sonar provided by the U.S. Navy Supervisor of Salvage provides us an unprecedented map of the seabed which will take some time to fully appreciate and analyze,” Mr. Terrill said.

Area coverage map of the 350 square kilometers (135 square miles) sonar survey of the two dump sites in the San Pedro Basin by an autonomous underwater vehicle. This figure shows the footprint of the 2021 and 2023 surveys, with the red lines indicating video image collection. (Courtesy of University of California–San Diego Scripps Institution of Oceanography)

The survey data follow ongoing studies by the University of California–Santa Barbara’s David Valentine, who in 2011 and 2013 discovered concentrated accumulations of DDT in the sediments and visually confirmed 60 barrels on the seafloor. Mr. Valentine is mapping DDT in sediments collected across the San Pedro Basin as part of the same project as the seafloor survey.

“Our preliminary findings of 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 Mr. Valentine, who in a 2019 study characterized the disposal of DDT waste as “inherently sloppy.” “Once dumped, DDT spread at the seafloor, expanding its footprint to at least the base of the Catalina slope. We are finding that original DDT remains abundant in the seafloor today, in both absolute and relative terms.”

The Environmental Protection Agency has also stated that the majority of DDT found at offshore dumpsites was likely deposited through bulk-dumping rather than containerized barrels, according to the UCSD researchers.

Example munitions surveyed by a remotely operated vehicle during a seafloor survey, led by the University of California–San Diego Scripps Institution of Oceanography, of the San Pedro Basin in 2023. (Courtesy of University of California–San Diego Scripps Institution of Oceanography)

Additionally, in the 2023 survey, scientists mapped whale falls, which are sunken whale carcasses. A total of seven whale falls were confirmed with video imagery, but the sonar data suggests more than 60 may exist in the footprint of the survey data collected by the autonomous underwater vehicle.

“The number of whale falls seems quite high relative to previous models of how many may occur on the seafloor off California,” said Scripps Oceanography marine biologist Greg Rouse. “However, the skeletons were mainly in very low-oxygen water that likely slowed decomposition markedly and the burial rate by sediment may also be very slow there. This would mean the whale falls may have accumulated over many decades.”

A sample image of a whale fall identified in the sonar survey of the San Pedro Basin. (Courtesy of University of California–San Diego Scripps Institution of Oceanography)