If all the salt dissolved in the oceans were removed and spread over dry land, it would form a layer more than 150 m thick, about as tall as a 40-storey building. Most of it comes from rocks: slightly acidic rain wears them down, and rivers carry the minerals to the sea. The water evaporates; the salt stays behind.
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.