Beneath Hawaii, a plume of hot rock rises from the mantle at an almost fixed spot. The Pacific plate slides over it, so each volcano is eventually carried away from its source, goes extinct and erodes. The result is a chain of islands and seamounts about 6,000 km long. The youngest island is still active, and a new volcano is already growing underwater to its southeast.
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.