Salvage crews have wrapped up pumping the hazardous oil-based substances from a sunken barge that apparently had been sitting undiscovered on the bottom of Lake Erie for nearly 80 years.

The U.S. Coast Guard said Thursday that all but two of the tanks on the barge turned out to be empty. Crews finished pumping an unknown cloudy substance from one of the tanks over the last few days.

The Coast Guard has been monitoring the site near the U.S.-Canadian border since discovering a small leak in October that appeared to be coming from a barge called the Argo that sank during a storm in 1937.

The wreckage is one of 87 shipwrecks on a federal registry that identifies the most serious pollution threats to U.S. waters.