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.
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 mid-ocean ridge snakes across the floor of every ocean for about 65,000 km, like the seam on a tennis ball. It is by far the longest mountain range on the planet, yet about 90% of it lies underwater. Along its axis, magma wells up and continuously builds new ocean floor.