No, No! Neither "tunnel" nor well need to withstand the pressure of 5000 feet of water. Reason is that the well (tunnel) is always filled with either water or drilling mud (roughly the same density as water) so the inside and outside pressure on the drilling pipe are the same. The pressure differences are not significant until/unless drilling into an oil or gas pocket. Even then the pocket must have a pressure significantly greater than that of the 5000 feet of water to be any problem. But that's what blowout preventers are for.

Don't forget that it's not only the lateral pressure you have to deal with, you have to have pipes that can not crumple under their own weight and simultaneously withstand extremely high torques due to ocean currents.

This is underwater remember - the weight thing is a solved issue, just include flotation along its length and its effective weight is as low as you care to make it.

There are few strong currents in the Gulf - it is a source for the Gulf Stream, but internal currents are fairly weak compared with those dealt with regularly in the oceans.

Is this a fact based on actual calculation/experiments, or your personal theory?

I don't think the solution is practical in this case for various other reasons, but the fact that offshore platforms exist incites me to think that long, underwater pipes can in fact resist ocean currents. They will also be naturally buoyant so long as there's enough air pockets in the material (or just floaters as the other person posted).

Personal theory. I should have said that in the original comment.

Even if the inside water pressure is the same as the outside water pressure, isn't there a great amount of pressure exerted on the pipe's structure simply by it's existence at such deep depths?

In other words, even if it's filled with water at 5000 feet of equal pressure, isn't there still a much greater pressure exerted on the pipe's walls at 5000 feet than there is at 100 feet?

Or am I misunderstanding something fundamental about water pressure?

You just need more numbers/sources. Intuition is a limited tool, especially in physics.

According to this ( http://www.calctool.org/CALC/other/games/depth_press ) the pressure would be around 15 MPa (this isn't hard to calculate by hand either). Consider that the mud/oil pushes 15 MPa, and water the same: that gives 30 MPa of purely compressive stress. Concrete could take this much ( http://www.cement.org/basics/concretebasics_faqs.asp ), and steel even more easily ( http://en.wikipedia.org/wiki/Tensile_strength -- if I'm not mistaken, steel behaves roughly the same in compression and in tension).