deep-level

Adjective

deep-level (not comparable)

1. Built or occurring underground at a relatively deep level.
   • Plans were prepared under the auspices of the L.S.W.R. by W. R. Galbraith and R. F. Church, with J. F. Greathead in consultation, for a deep-level electric line on the tube principle, 1½ miles long, from a point under the main-line terminus at Waterloo to the eastern end of Queen Victoria Street...1958 August, T. S. Lascelles, “Diamond Jubilee of the Waterloo & City Railway”, in Railway Magazine, page 517
   • The deep-level Tubes required the invention of a tunnelling machine and a means of propelling trains through those tunnels that would not choke the passengers. The answer to the latter was electricity, and we have seen how electric traction had become established by the late nineteenth century.2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 86