At about 370,000 km², the Caspian Sea is the largest body of water entirely enclosed by land on the planet. With no connection to the ocean, geographers classify it as a lake, even though its water is slightly salty, about a third as salty as seawater. It is a remnant of an ancient ocean, the Tethys, cut off by moving plates.
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.