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, using the cut and cover method, and are large enough to take trains of normal size. Later, smaller, roughly circular tunnels--which gave rise to its nickname the Tube--were dug through the London Clay at a deeper level.