Over deep water, a tsunami can race along at up to about 800 km/h, the speed of an airliner. Yet out at sea it is often barely a metre high, so a ship can ride over it without noticing. Only near the coast does it slow down, piling its energy into a wave that can reach tens of metres.
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.