For decades, divers and deep-sea explorers off the coast of Los Angeles have reported eerie ghostly halos glowing faintly around corroding barrels on the seafloor.
Long dismissed as microbial mats or unexplained anomalies, these ghostly halos have now been identified as the lasting footprint of a toxic dumping practice that began more than half a century ago.
A new study published in Proceedings of the National Academy of Sciences Nexus by researchers at UC San Diego’s Scripps Institution of Oceanography reveals that the halos mark barrels filled not with the infamous pesticide DDT, but with caustic alkaline waste.
Leaking from steel drums dumped in the San Pedro Basin in the 1950s and ’60s, the waste has altered the very chemistry of the seafloor, creating conditions so extreme they resemble deep-ocean hydrothermal vents.
“This adds to our understanding of the consequences of the dumping of these barrels,” marine microbiologist at Scripps and senior study author, Dr. Paul Jensen, said in a press release. “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.”
From the 1930s through the early 1970s, Southern California’s booming chemical and oil industries legally discharged massive amounts of industrial waste into the ocean.
The Montrose Chemical Corporation, once the nation’s largest DDT manufacturer, produced more than 800,000 tons of the insecticide and offloaded thousands of gallons of contaminated sludge into Los Angeles’s sewers and directly into the Pacific. Barge operators also ferried containerized waste to designated “dumpsites,” rolling steel drums into the deep, sometimes after puncturing them so they would sink.
By some estimates, more than 300,000 barrels were discarded in the San Pedro Basin. A 2023 sonar survey later identified about 27,000 “barrel-like” objects on the seafloor, many badly corroded and some encircled by the mysterious glowing white ghostly halos.
For decades, the assumption was that these containers were a primary source of DDT contamination, which continues to plague marine life and has been detected in everything from fish off the Palos Verdes Shelf to bottlenose dolphins. However, results from this latest research complicate that picture.
Using remotely operated vehicles (ROVs), researchers collected sediment cores from barrels surrounded by halos and from control sites without them. To their surprise, DDT and its breakdown products, collectively referred to as DDX, were elevated across the entire dumpsite. However, the contamination did not spike near the barrels themselves. Instead, the halos revealed something stranger.
The sediments inside the rings were chemically different, with unusually high pH values bordering on the limits of life. Minerals such as calcite and brucite, typically seen in exotic geological settings like hydrothermal vents, were precipitating in the mud.
DNA sequencing showed that the microbial communities inside the halos were unique, dominated by rare extremophiles adapted to hyperalkaline conditions. This discovery adds a new dimension to our understanding of these ecosystems.
“We show that alkaline waste pollution continues to alter microbial sediment communities more than 50 years after initial disposal and can be considered a persistent pollutant that selects for highly specialized microbial communities related to those observed in hydrothermal seeps and other hyperalkaline habitats,” researchers write.
In essence, leaking alkaline waste has transformed parts of the San Pedro Basin into miniature versions of natural deep-sea vents. Brucite concretions form near the barrels, slowly dissolving and releasing hydroxides that drive pH upward. As seawater mixes with these fluids, calcium carbonate precipitates at the edges, producing the white ghostly halos that caught explorers’ attention in the first place.
“These visual cues provide a rapid method to predict which barrels contained alkaline waste, which account for ca. one-third of the steel barrels visually identified to date on the San Pedro Basin seafloor,” the study notes.
Rather than dispersing into the vastness of the ocean, the alkaline waste persists in solid mineral form, leaching slowly into surrounding sediments. The process could continue for “several thousands of years,” researchers warn, making these dumpsites not just historical scars but active, evolving ecosystems of pollution.
These findings shift the narrative about LA’s underwater dumps. While DDT remains a well-documented concern, with residues still found in fish and linked to human health impacts across generations, the barrels themselves appear to have been used for other types of waste. Some likely carried caustic industrial byproducts that now act as chemical reactors on the ocean floor.
The ecological consequences are only beginning to be understood. Sediments around the ghostly halos host fewer species and lower diversity than the surrounding areas. That reduction cascades upward, affecting worms, crustaceans, and fish that rely on microbial productivity as the base of the food chain.
Perhaps most striking, these artificial alkaline hotspots are so unusual that they more closely resemble deep-subsurface aquifers and serpentinite springs in Italy than the typical sediments lying just a few meters away. What was once seen as a “dead zone” of pollution is, in fact, a bizarre laboratory where evolution is selecting for microbes that can thrive at extreme pH levels.
The study underscores the long tail of industrial practices carried out when ocean dumping was not only legal but encouraged as a cheap disposal method.
Between 1961 and 1964 alone, the Pacific Ocean Disposal Company dumped nearly 1.4 million gallons of alkaline waste in the San Pedro Basin. More than half a century later, the chemical fingerprints remain, altering life on the seafloor in ways no one predicted.
Addressing the San Pedro Basin’s legacy pollution will not be easy. Removing the toxic barrels is technically challenging and prohibitively expensive. Mapping and monitoring may be the most realistic path forward, enabling researchers to track changes over time and more accurately assess risks to marine ecosystems and coastal communities.
Additionally, scientists caution that the San Pedro Basin may hold more surprises. The discovery of alkaline waste lurking inside barrels once assumed to contain only DDT underscores the incompleteness of our understanding of these dumpsites.
With tens of thousands of debris targets scattered across the seafloor, many still unexamined, researchers still do not know the true scope of what was discarded during Southern California’s industrial boom.
“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,” Scripps postdoctoral scholar and co-author, Dr. Johanna Gutleben, said. “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.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com