In places such as the Gulf of Mexico and the Red Sea, brines several times saltier than seawater pool in hollows of the seafloor. Too dense to mix, they form true underwater lakes, with a surface, shorelines and even waves. Starved of oxygen, they are deadly to most animals that wander in.
On 27 August 1883, the eruption of the Indonesian volcano Krakatoa was heard as far away as Rodrigues Island, nearly 4,800 km away. Its pressure wave was picked up by barometers worldwide and circled the globe several times. The tsunamis that followed killed more than 36,000 people.
The deepest known point in the ocean, Challenger Deep, lies roughly 10,900 m below the Pacific surface. Drop Everest in and its summit would still sit about 2 km underwater. Down there, the pressure is more than 1,000 times what you feel at sea level, roughly a tonne pressing on every square centimetre.