TheDove wrote:Sooo... what exactly is the difference between water rushing in and air rushing out?
The air doesn't rush out. It gets compressed and pushed further in. Normally you'd expect air to rush out until the pressure is equalized, then for air to continue leaving while water enters. At that depth, no air leaves, but water still enters.
It's.. not just that there's 10 pounds of shit in the 5 pound bag, it's that there's now an extra 30 pounds of shit being crammed in on top of the 10 pounds in the 5 pound bag.
Seconded. Here's another way to think about it: Imagine that you're the scuba diver, with lungs full of air at the same pressure as the water around you. If you dive below this line then when you open the flow valve from the tank the air in your lungs would get compressed into the canister instead of flowing out like you'd want it to. That line is therefore the theoretical maximum scuba depth, based on Randal's research into scuba tanks. There are other practical limits, like oxygen and nitrogen poisoning, or the anesthetic effect of noble gasses like Helium; these explain why the scuba record is nowhere near the tank pressure line.
I'm amusing myself, however by imagining that the pressure limit were the only one we were concerned with. A scuba tank could be charged far beyond its rated surface capacity if we could pre-position it at great depth and fill it on-site using hoses from the surface. Such a tank would explode if brought to the surface without depleting the air, but as long as it stayed submerged the water pressure would compensate for the extra air pressure keeping it safe for use during the dive. And now I can rest contented that I've solved a problem that no one will likely ever encounter