Strange “Halos” on the Ocean Floor off Los Angeles Reveal a Toxic Secret
Once believed to contain the pesticide DDT, new analysis shows some barrels actually held caustic alkaline waste. In 2020, striking photographs revealed rusted barrels scattered across the seafloor near Los Angeles, capturing widespread attention. At first, the corroded containers were suspected to hold residues of the pesticide DDT, especially since some were surrounded by pale, [...]
A discarded barrel on the seafloor off the coast of Los Angeles. The image was taken during a survey in July 2021 by remotely operated vehicle SuBastian. Credit: Schmidt Ocean InstituteOnce believed to contain the pesticide DDT, new analysis shows some barrels actually held caustic alkaline waste.
In 2020, striking photographs revealed rusted barrels scattered across the seafloor near Los Angeles, capturing widespread attention. At first, the corroded containers were suspected to hold residues of the pesticide DDT, especially since some were surrounded by pale, halo-like rings in the sediment. Yet the actual contents of the barrels, as well as the cause of the strange halos, remained uncertain.
Research led by UC San Diego’s Scripps Institution of Oceanography has since clarified that the halo-producing barrels contained caustic alkaline waste, which seeped out and altered the surrounding environment. While the study could not determine the precise compounds inside, it noted that DDT production produced both alkaline and acidic byproducts. In addition, other major industries in the area, including oil refining, were known to release large amounts of alkaline waste.
“One of the main waste streams from DDT production was acid, and they didn’t put that into barrels,” said Johanna Gutleben, a Scripps postdoctoral scholar and the study’s first author. “It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?”
Researchers use Remotely Operated Vehicle SuBastian to collect sediment push cores next to barrels discarded on the seafloor. Credit: Schmidt Ocean InstituteToxic transformations of the seafloor
The research showed that leaking alkaline waste reshaped parts of the seafloor into harsh habitats resembling natural hydrothermal vents — environments that host specialized microbes capable of surviving where most organisms cannot. According to the study’s authors, the scale and intensity of these impacts on marine ecosystems depend on both the number of barrels resting on the seafloor and the particular chemicals they released.
Even with these uncertainties, Paul Jensen, a Scripps emeritus marine microbiologist and the study’s senior author, explained that he had assumed such alkaline material would quickly dilute in seawater. Instead, it has remained intact for more than fifty years, leading him to conclude that this waste “can now join the ranks of DDT as a persistent pollutant with long-term environmental impacts.”
Released on September 9, 2025, in the Proceedings of the National Academy of Sciences Nexus and backed by NOAA along with the University of Southern California’s Sea Grant program, the study adds to Scripps’ longstanding efforts to investigate the toxic legacy of once-permitted dumping in Southern California’s offshore waters. The results also offer a visual method to distinguish barrels that once carried this alkaline waste.
“DDT was not the only thing that was dumped in this part of the ocean and we have only a very fragmented idea of what else was dumped there,” said Gutleben. “We only find what we are looking for and up to this point we have mostly been looking for DDT. Nobody was thinking about alkaline waste before this and we may have to start looking for other things as well.”
VIDEO
Video footage from ROV SuBastian’s exploration around the DDT Barrel Site 1 in the Southern California Borderland off the coast of Los Angeles. Credit: Schmidt Ocean Institute
Ocean dumping legacy in California
From the 1930s through the early 1970s, 14 deep-water dumping grounds off the coast of Southern California were used to dispose of “refinery wastes, filter cakes and oil drilling wastes, chemical wastes, refuse and garbage, military explosives and radioactive wastes,” according to the EPA. Seafloor surveys led by Scripps in 2021 and 2023 documented thousands of discarded objects, including hundreds of military munitions. The total number of barrels lying on the ocean floor is still unknown. Sediments in this region are heavily contaminated with DDT, a pesticide banned in 1972 and now recognized as dangerous to both humans and wildlife. Sparse records from the period suggest that most DDT waste was discharged directly into the sea.
Gutleben said she and her co-authors didn’t initially set out to solve the halo mystery. In 2021, aboard the Schmidt Ocean Institute’s Research Vessel Falkor, she and other researchers collected sediment samples to better understand the contamination near Catalina. Using the remotely operated vehicle (ROV) SuBastian, the team collected sediment samples at precise distances from five barrels, three of which had white halos.
The barrels featuring white halos presented an unexpected challenge: Inside the white halos the sea floor suddenly became like concrete, preventing the researchers from collecting samples with their coring devices. Using the ROV’s robotic arm, the researchers collected a piece of the hardened sediment from one of the halo barrels.
Paul Jensen and Johanna Gutleben of UC San Diego’s Scripps Institution of Oceanography unload and sort sediment cores after the samples were brought to the surface from known dumping sites by Remotely Operated Vehicle (ROV) SuBastian during a July 2021 expedition aboard Research Vessel Falkor. Credit: Schmidt Ocean InstituteTesting for DDT and microbes
The team analyzed the sediment samples and the hardened piece of halo barrel crust for DDT concentrations, mineral content and microbial DNA. The sediment samples showed that DDT contamination did not increase closer to the barrels, deepening the mystery of what they contained.
During the analysis, Gutleben struggled to extract microbial DNA from the samples taken through the halos. After some unsuccessful troubleshooting in the lab, Gutleben tested one of these samples’ pH. She was shocked to find that the sample’s pH was extremely high — around 12. All the samples from near the barrels with halos turned out to be similarly alkaline. (An alkaline mixture is also known as a base, meaning it has a pH higher than 7 — as opposed to an acid which has a pH less than 7).
This explained the limited amount of microbial DNA she and her colleagues had been able to extract from the halo samples. The samples turned out to have low bacterial diversity compared to other surrounding sediments and the bacteria came from families adapted to alkaline environments, like deep-sea hydrothermal vents and alkaline hot springs.
Analysis of the hard crust showed that it was mostly made of a mineral called brucite. When the alkaline waste leaked from the barrels, it reacted with magnesium in the seawater to create brucite, which cemented the sediment into a concrete-like crust. The brucite is also slowly dissolving, which maintains the high pH in the sediment around the barrels, and creates a place only few extremophilic microbes can survive. Where this high pH meets the surrounding seawater, it forms calcium carbonate that deposits as a white dust, creating the halos.
Photo of DDT Dumpsite Sediment core processing team. From left to right: Kira Mizell (USGS), Johanna Gutleben, Paul Jensen, Devin Vlach, Michelle Guraieb, Lisa Levin. Credit: Brady Lawrence for Schmidt Ocean InstituteLasting ecological consequences
“This adds to our understanding of the consequences of the dumping of these barrels,” said Jensen. “It’s shocking that 50-plus years later you’re still seeing these effects. We can’t quantify the environmental impact without knowing how many of these barrels with white halos are out there, but it’s clearly having a localized impact on microbes.”
Prior research led by Lisa Levin, study co-author and emeritus biological oceanographer at Scripps, showed that small animal biodiversity around the barrels with halos was also reduced. Jensen said that roughly a third of the barrels that have been visually observed had halos, but it’s unclear if this ratio holds true for the entire area and it remains unknown just how many barrels are sitting on the seafloor.
The researchers suggest using white halos as indicators of alkaline waste could help rapidly assess the extent of alkaline waste contamination near Catalina. Next, Gutleben and Jensen said they are experimenting with DDT contaminated sediments collected from the dump site to search for microbes capable of breaking down DDT.
The slow microbial breakdown the researchers are now studying may be the only feasible hope for eliminating the DDT dumped decades ago. Jensen said that trying to physically remove the contaminated sediments would, in addition to being a huge logistical challenge, likely do more harm than good.
“The highest concentrations of DDT are buried around 4 or 5 centimeters below the surface — so it’s kind of contained,” said Jensen. “If you tried to suction that up you would create a huge sediment plume and stir that contamination into the water column.”
Reference: “Extremophile hotspots linked to containerized industrial waste dumping in a deep-sea basin” by Johanna Gutleben, Sheila Podell, Kira Mizell, Douglas Sweeney, Carlos Neira, Lisa A Levin and Paul R Jensen, 9 September 2025, PNAS Nexus.DOI: 10.1093/pnasnexus/pgaf260
This research was funded by the National Oceanographic and Atmosheric Administration award nos. NA23NMF4690462 and NA22OAR4690679 to P.R.J. and L.A.L. and the University of Southern California Sea Grant award SCON-00003146 to L.A.L.
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