Mysterious white halos emerging around sunken barrels of chemical waste on the seafloor off California's coast have been found to contain traces of an alkaline substance, providing a tantalizing clue to their origins.

Thousands of containers with unknown contents were dumped into the Pacific off the coast of Los Angeles, near Catalina, in the 20th century. In the past decade, researchers manning remote underwater robots have repeatedly come across their corroding remains.

A recent sonar survey detected around 27,000 of the barrels scattered across the San Pedro basin – a mere fraction of the estimated half-million that may have been dumped by the DDT industry, particularly the Montrose Chemical Company.

Testing has repeatedly shown the noxious insecticide is still present in the seafl

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