Pressure: Into the deep?
When we head out into the ocean or the water of any kind it helps to remember that it's still the same concept. The pressure in the ocean is just the weight of all the air above the ocean plus all the ocean above you at the time. It's pretty helpful to remember that water is pretty much 1000 times as dense as air. That means that 1 m of water (3 ft) feel like 1000 m or 1 km of air. 10 m of water adds 1 atm of pressure. If you want to send a piece of equipment down to examine the floor of the ocean (usually about 2 miles or 3000 m) it has to be able to not crush when subjected to 300 atm of pressure -- that's just ridiculous! Going to the bottom of the Marianas Trench (11,000 m) is like trying to get a space probe to land on the surface of Jupiter or Saturn. Not surprisingly when we try this the probe makes it a ways and then is crushed and is gone.
It's very thoughtful to ask how creatures manage to live in the depths of the ocean where the pressures are so extreme (never mind the dark). Part of their adaptation is to have no organs that contain air that can be squeezed by the tremendous pressure. This is related to why you can have thousands of kg (lbs) of air sitting on you all the time and not be crushed. The air in your lungs pushes outward on your insides as hard as the air outside pushes inward on your. The rest of your body is a bag of salty water and water doesn't compress. One reason it is hard to scuba dive to significant depths is that the pressure in your lungs has to match the pressure of the water to keep things balanced and not crush your chest. Your lungs are not well adapted to dealing with really high pressure air. This is why some proposals for diving to great depths suggest 'breathing' high oxygenated water instead of air. A bit of a terrifying thought but you can begin to see why.
