[planetguy]

There are two problems here.

Firstly: As I see it, an evacuated tunnel at 20m is a very different proposition to a non-evacuated tunnel at 30m, even if the pressure difference is the same. Why? Well, your big problem is always going to be leaks. In an air-filled tunnel you can get away with microscopic cracks, no problem; you've got a small direct interface between water and air, and the surface tension of the water is enough to keep the water in place. It's very hard to force water through a really tiny crack. In a vacuum, however, surface tension goes away -- liquid water at an interface with vacuum will boil, and all of a sudden you've got water vapour filling your nice evacuated tube through every microscopic crack in its five thousand mile length. Nasty. Clearly the joined-concrete construction used for the Transbay Tube isn't going to be sufficient.

Secondly: unfortunately the Atlantic Ocean isn't 20m deep, or 30m deep, or even 40m deep like the deepest point of the Transbay Tube. It's several kilometers deep.

[allenlavoie]

Is there some reason that sandwiching air between the evacuated tube and the water wouldn't work? Granted it would add to the cost.

The ocean depth point is interesting. The article says "engineers would tether the tunnel at a fixed depth." I take this to mean tethering to the bottom and relying on buoyancy to keep the tunnel floating at the right depth, which presumably could be 30m or so.

[planetguy]

>Is there some reason that sandwiching air between the evacuated tube and the water wouldn't work?

Yep, because now you've just got your air rushing into the vacuum through the cracks, instead of water.

The ocean depth point is interesting. The article says "engineers would tether the tunnel at a fixed depth." I take this to mean tethering to the bottom and relying on buoyancy to keep the tunnel floating at the right depth, which presumably could be 30m or so.

Even if you could get it to be neutrally bouyant at 30m (realistic estimate? dunno) you're now stuck with a tube, five thousand miles long, floating free in the ocean and anchored only to the bottom. Even neutrally bouyant, there'd have to be huge strains on the joints due to ocean currents, plus presumably some up-and-down forces as the season changes the water temperature and... heck, even a whale headbutting such a flimsy structure sounds like a disaster waiting to happen. And remember, you can't afford to get any imperfections in your tube or it'll leak -- that probably eliminates anything you might have used to build in a bit of flexibility.

[allenlavoie]

>Yep, because now you've just got your air rushing into the vacuum through the cracks, instead of water.

Fair enough. I imagine there's a trade-off between the quality of the tunnel materials and the number of pumps needed along the way, assuming an imperfect vacuum is good enough.

I'd be worried about flexibility too. Skyscrapers do pretty well in high winds and earthquakes, so I'm not completely convinced that a similar effect isn't possible with an underwater tube. Any bending would have to be extremely gradual, though, if there's any hope of shooting something through it at high speeds. I agree that glass isn't going to cut it.

[rfugger]

Some kind of gel between the layers perhaps?