At the surface, tropical water often tops 28 °C. But a few hundred metres down, the temperature plunges, and in the deep ocean it hovers around 0 to 4 °C whatever the latitude. These cold, dense waters originate in the polar regions, where they sank. Most of the ocean's volume is thus about as cold as your fridge.
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.