In the Arabika massif of the Caucasus, Veryovkina and Krubera-Voronya caves plunge more than 2,200 m below their entrances, with the title of deepest shifting as new explorations come in. Reaching the bottom takes weeks of rope descents, dives through icy sumps and underground camps. Cavers have been trapped down there by sudden floods.
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.