Spanning 3,400 miles, the tunnel would be the most ambitious infrastructure project ever undertaken. The difficulties of building an underground rail tunnel all the way from London to New York might seem insurmountable, yet plans to actually construct such a tunnel are genuinely being considered, with those behind the idea estimating that it could cost somewhere in the region of $20 trillion. Not only would the tunnel be the longest ever built by a huge margin at 3,400 miles long, but it would also make incredibly fast trans-Atlantic travel a reality, with trips between New York and London taking a...