system
Given the following post, respond as if you're commenting on Reddit:
human
Is a transatlantic bridge/tunnel possible? Why or why not?
gpt
They are really two different questions (cue ELI5 mode):

**Bridge:**

The Atlantic is really really big.  Based on where you cross, it could even be bigger than the USA.  On top of that, it's wet and it gets storms which both can cause quite a bit of damage to concrete and steel which would be used to make the bridge.

The ocean is too deep to sink the pilings down to anchor the bridge.  It's too stormy and full of waves so that a pontoon type bridge would just shake itself to pieces.  

It's also big and empty and very few people would really want to drive across it because it would be very boring and they wouldn't be able to carry enough fuel for the trip.

(Adult version:  we can't sink piling to stabilise the bridge and current structural materials won't stand up to the stresses of a pontoon bridge.  Maintenance would be an absolute nightmare due to the corrosion of salt water/air.  You could only refuel on the crossing if fuel were shipped out to the middle of the ocean, which basically defeats the purpose)

**Tunnel:**

The Atlantic is very very deep.  All of that water on top of the ocean floor means that there's lots of pressure at the bottom.  Kind of like how your ears hurt when you dive deeply at the pool but multiple it by 10,000.

All of that pressure means that if they tried to dig a tunnel then it would have to absolutely never ever spring a leak or the walls would crumble down and everyone would drown.

On top of that, you'd need really really big fans to make sure that there was enough air for everyone down there.  You would also have the same problem with fuel.

(Adult version:  the real problem would be at the joins of the pre-fabricated tunnel sections which would go into the tunnel hole.  There's no way that they'd be able to be made strong enough to withstand the pressure and even small leaks at that pressure would penetrate materials like a movie laser.  On top of that, ventilation of the tunnel would be an enormous issue.  You would have thousands of kilometers of dead air that would have to be circulated somehow or you'd never make it through.  As above, fuel as well)