Trivia About the Circle Line

Technically it is in fact a 'sub-surface' line, running not much deeper than the basement of surrounding buildings. It was built by the 'cut and cover' method, where a deep trench is dug and then covered over. Along some parts of the line the trench was left open, and the sky is plainly visible from some sections of the Circle Line as well as from a number of stations along its route. Compare this to the later 'deep level' tubes such as the Piccadilly and Northern Lines, which run much deeper and were dug using tunnelling machines.

from http://www.bbc.co.uk/dna/h2g2/A592995