No one has ever been to Earth's centre: everything we know about it comes from seismic waves passing through it. Studying earthquake signals that appeared where they should not have, Danish seismologist Inge Lehmann deduced in 1936 that the liquid core contains a solid inner core. Her idea, published in a paper only a few pages long, was confirmed decades later.
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.