• Underground aquifer
• Underground bunker
• Underground casemate
• Underground catacomb
• Underground catacombs
• Underground cesspool
• Underground cistern
• Underground clink
• Underground coalmine
• Underground conventicle
• Underground crawlies
• Underground crypt
• Underground economy
• Underground facility
• Underground formicary
• Underground garage
• Underground grotto
• Underground hideout
• Underground hypogeum
• Underground ice-house