The system's first tunnels were built just below the surface, using the cut and cover method; later, smaller, roughly circular tunnels--which gave rise to its nickname, the Tube--were dug through at a deeper level.
The system's first tunnels were built just below the surface. The system's first tunnels were using the cut and cover method. Smaller, roughly circular tunnels--which gave rise to its nickname, the Tube--were dug through at a deeper level. This was later.