Continents hold rocks nearly 4 billion years old. The ocean floor, by contrast, is rarely older than 180 to 200 million years. Born at mid-ocean ridges, it gradually moves away and then sinks back into the mantle at subduction zones, where it is recycled. Oceans thus renew their floors entirely, while the lighter continents stay afloat.
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.