The Mersey Tunnel...How did they build it?

[Wacky Racer]

Built in the 1930's, the Mersey Tunnel was, and is still considered a masterpiece of civil engineering, but how was it built?

It twists and turns throughout it's length, and even has inclines and declines, unlike many rail tunnels which are dead straight,... how did they know where it was going to emerge?...They could not even be guided by vertical ventilation shafts, as in the Woodhead and Stanedge pennine rail tunnels, because of the million of tons of water above it.

http://www.youtube.com/watch?v=CVJd2etn44M