According to the US Geological Survey, more than 80% of Earth's surface, on land and under the sea, was formed by volcanic activity. Most of it is out of sight: the entire ocean floor is born from lava welling up along mid-ocean ridges. The volcanoes you know are just the visible tip of a vast crust factory.
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.