Before the Industrial Revolution, the air held about 280 parts per million (ppm) of CO₂. Today it holds more than 420. According to the IPCC, concentrations this high have not occurred for at least 2 million years, long before our species appeared. The last time Earth had comparable levels, around 3 million years ago, sea level was several metres higher.
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.