Just like there are waterfalls underneath oceans, there can be lakes on seabed too. One is kept separate from the surrounding body of water by density arising from temperature difference, while the other is kept separate by it’s salinity. These saline lakes of dense water on the seafloor has distinctive shorelines and surfaces, and they are called brine pools. They are common in the Gulf of Mexico, where the smallest ones are not much bigger than a puddle, measuring just one meter across, but the biggest pools are up to 20 km long.