Somewhere around 600 to 1,200 m deep, depending on the region, temperature and pressure combine to create a layer where sound travels slowest. Sound waves get trapped there, like light in an optical fibre, and can travel thousands of kilometres. This 'SOFAR channel' helps detect undersea earthquakes, and some whales may use it to communicate.
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.