Howell and her colleagues found all of their study locations had some trash. The most litter-filled locations had more than 20 pieces of trash per hectare (about 2.5 acres). The cleanest places were along continental shelves and had just one or two items per hectare. Plastic was the most common litter material the HERMIONE researchers found. Scientists worry about plastic in the ocean because it can release chemicals into the water that are toxic to marine creatures. The HERMIONE researchers also saw a lot of abandoned fishing gear. These stray lines and nets can catch and kill sea life even though their human owners are no longer using them. The researchers even saw clinker, the burnt remnants of the coal that 18th-century steamships used to burn. Steamship crews regularly dumped clinker overboard. It's hung out on the ocean floor since.