JDFonz, that salt mine complex is awesome, very artistic.
Having run out of "above grade" space, I too have contemplated modeling the underworld; and, I thought of modeling an underground cityscape, a small area however that could only be about 2' x 3'. The idea was motivated by Seattle's underground city, ostensibly buried during a late 19th century earthquake, and they built a new city on top. Kolchak's Night Strangler episode also inspired me, the scenes of the underground were eerie.
While not exactly under my layout, I am currently working on building a seashore scene under a long truss bridge...it's taken me all summer and am finally seeing it to completion.