Ice is slightly less dense than seawater: about 917 kg/m³ versus 1,025. An iceberg therefore floats with nearly nine-tenths of its volume underwater. The visible part really is just the tip, and the hidden mass, with its irregular shape, can extend far to the sides, which is what made icebergs so dangerous to ships.
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.