deep-level

Adjective

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 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...