relief well: how can they intersect with the other pipe, 5 miles below water level?

[Mr Downtown]

Laser beams allow some amazing things. The Chunnel was dug from both ends and had only a couple centimetres difference in alignment when they met.

But surveyors have always been pretty amazing. One of Chicago's first water supply tunnels was dug under the surface of Lake Michigan. One crew started on shore and another crew started from the water intake crib two miles out in the lake. Crews of Irish workmen dug by gaslight from both ends, lining the tunnel with brick as they went. When they met, they were four inches off. In 1863.