One at a time. We’ve actually already answered a question about pressure on submarines, so check it out: Submarines and Water Pressure
As for planes, the important thing to understand is Bernoulli’s Principle. What that says is that the faster air is moving, the less pressure it has. In other words, if the air is moving really fast, it doesn’t push on things as hard. This is where the shape of the plane wing comes in. The plane wing is basically flat on the bottom and has a pretty big curve around the top. So the air that hits the edge of the wing gets split - some goes up over the wing and some goes down under.
The air going over has to go a lot farther than the air going under, since the top edge is curved. And it has to get there in the same amount of time. (If the air on the top took more time, then there would be an open space with no air in it, which doesn’t happen.) So the air over the top actually has to go /faster/ than the air on the bottom. Since the air on the top is going faster, it has lower pressure, so it doesn’t push as hard as the slower air on the bottom. So the wing gets pushed up more than it gets pushed down, and the plane goes up.
(republished on 07/30/06)