deep-level

deep-level (not comparable)

1. Built or occurring underground at a relatively deep level.
   • 2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 86:

     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.

• high-level
• low-level