In the North Atlantic, cold, salty surface water grows dense and sinks into the abyss. It then begins a slow journey around the globe known as the thermohaline circulation. A given parcel of water is estimated to take around 1,000 years to complete the loop, so some water surfacing today hasn't seen the sky in a millennium.
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.