On 21 August 1986, Cameroon's Lake Nyos, which sits in a volcanic crater, suddenly released a huge cloud of carbon dioxide that had built up in its deep waters. Heavier than air, the gas flowed down the valleys and suffocated about 1,700 people and thousands of animals. Pipes installed in the lake now vent the gas gradually to prevent another 'limnic eruption'.
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.