In the Denmark Strait, between Greenland and Iceland, frigid, dense water spills over an undersea ridge and plunges down the slope for more than 3 km. Its flow is estimated at about 5 million cubic metres per second, far more than any waterfall on land. Invisible from the surface, it is the largest waterfall known on Earth.
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.