Oil spills have caused plenty of marine disasters, so this may come as a shock: ships used to leak oil into the water on purpose. The crews of these 19th-century ships dumped oil overboard to calm violent waters and make for a smoother, easier journey. Oil can have this effect on choppy waves because it forms a very thin film atop the water. The slippery layer makes it harder for the air to grab onto water molecules and drag them around with the wind to create waves. Instead, the air slips over the oil without much to tug around. The practice is not done today for obvious environmental reasons, but it's quite effective in calming violent seas.