Tunnel hole-through

Tunnel hole-through, also called breakthrough, is the time, during the construction of a tunnel built from both ends, when the ends meet, and the accuracy of the survey work becomes evident. Many tunnels report breakthroughs with an error of only a few inches, for example:

the 1858 Blue Ridge Tunnel – the 1944 Blue Ridge Tunnel replacement – - 1944 the Lötschberg Base Tunnel – 10 cmLotschberg the Holland Tunnel - 1 cm

See also Cascade Tunnel Siloam inscription