A benthic trawl is an enormous net dragged across the ocean floor. The net is held aloft with large floats, and kept down with a heavy steel trawl door and weights. The pocket of the net is known as the "codend." The drag marks left behind from the trawl are caused by the rollers, up to five tonnes each, which Bring up a dust cloud from the sea floor that can last for days. Benthic trawling has been used for over 500 years.