The Antarctic ice sheet holds about 90% of the planet's ice and nearly 70% of its fresh water. Over 2 km thick on average, it exceeds 4.5 km in places. If it melted entirely, which would take thousands of years, sea level would rise by about 58 m, enough to drown most of the world's major coastal cities.
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.