Seventy miles off the coast of Louisiana, among a maze of drilling platforms and seafloor pipelines, thousands of 55-gallon drums containing hazardous industrial chemicals litter a vast, dark swath of the ocean floor. Theyve been sitting there for nearly 50 years.

Charles McCreery was a few months into a new job as an oceanographer and water quality expert at the federal Bureau of Ocean Energy Management when he first learned of the dumping ground. It was 2014, and he was tasked with reviewing oil giant Shells exploration plans in an offshore leasing area known as Mississippi Canyon, in the north-central Gulf of Mexico. Deep in the document, he came across the companys internal policy for steering clear of toxic waste barrels, and what to do should their operations puncture or disturb one.

The content, and its toxicity, of each individual barrel is not known, the Shell document reads. Within the area there are/could be many hundreds of waste barrels. Many of the barrels may have released their contents over time.

The plan detailed potential hazards associated with the drums, from worker exposure while retrieving deep-sea equipment to underwater explosions. Metal salts, one of the many waste materials discarded at the site, react violently when exposed to water.

Starting in 1973, the Environmental Protection Agency issued chemical giants permits to discard thousands of drums of industrial chemical waste at the offshore site. The pollutants included chlorinated hydrocarbons, or CHCs, a family of toxic chemicals that can persist in the environment and become concentrated in marine organisms, potentially migrating up the food chain and posing a risk to human health.

In the decades since, oil companies have built up a vast network of wells and seafloor pipelines in the same portion of the Gulf. The areas largest producer is Shell Offshore Inc., a subsidiary of oil giant Royal Dutch Shell, which operates three platform rigs and three drillships in whats known as the Mars-Ursa oil basin. Shell also happens to be one of the companies that received permits from the EPA to dump huge quantities of industrial chemical waste in the Gulf in the 1970s, albeit at a different location.