[mprev]

Isn't the issue the depth of the Irish Sea at the point where Scotland and Ireland are closest? It gets to around 1,000 ft deep there (roughly 5x where the Channel Tunnel goes) and has a bunch of unexploded WW2 bombs dumped there, too.

Then Dublin to Holyhead is, what, 70 miles? That's twice as long as the Seikan tunnel, which I think is the world's longest under sea tunnel.

[rbanffy]

I think it isn’t that deep - last time I checked it was reasonable, but you make a good point about unexploded ordnance. Building it would be a thrill.