Name a type of structure built underground (man-made)

The rarest possible answer — almost no player finds it.

• Denehole— Chalk miners sank a narrow shaft, then hollowed out chambers below.
• Erdstall— A medieval tunnel network with tight squeezes and a still-debated purpose.

Genuinely obscure. Most players never think of it.

• Archive vault
• Artificial cave
• Auxiliary Units operational base
• Barabara
• Biogas digester
• Canal tunnel
• Catch basin
• Cist
• Columbarium
• Cryptoporticus
• Cycling tunnel
• Duct bank
• Earth lodge
• Flood tunnel
• Fogou
• Freight tunnel
• Geothermal well
• Ghost station
• Grease trap
• Hiding complex
• Hypocaust
• Hypogeum
• Infiltration gallery
• Inspection pit
• Leach field
• Mikveh
• Mithraeum
• Nilometer
• Ondol
• Ore pass
• Piles
• Pit latrine
• Pumped storage cavern
• Qargi
• Qarmaq
• ROC post
• Rock-cut church
• Rock-cut temple
• Rock-cut tomb
• Sardab
• Seismic vault
• Shaft tomb
• Slurry pit
• Soakaway
• Souterrain
• Stepwell
• Stope
• Storage cavern
• Stormwater tank
• Sump
• Sunken courtyard yaodong
• Sunken tandoor
• Tram tunnel
• Undercroft
• Underground aquarium
• Underground bathhouse
• Underground boiler room
• Underground bus station
• Underground cinema
• Underground dam
• Underground data centre
• Underground factory
• Underground farm
• Underground fortress
• Underground funicular
• Underground garden
• Underground granary
• Underground greenhouse
• Underground hangar
• Underground hospital
• Underground hotel
• Underground library
• Underground magazine
• Underground museum
• Underground nightclub
• Underground office
• Underground postal railway
• Underground power station
• Underground public toilet
• Underground pumping station
• Underground quarry
• Underground railway depot
• Underground reservoir
• Underground restaurant
• Underground sauna
• Underground school
• Underground sewage treatment plant
• Underground shooting range
• Underground sports hall
• Underground storage tank
• Underground street
• Underground submarine base
• Underground substation
• Underground telephone exchange
• Underground theatre
• Underground warehouse
• Underground water tank
• Underground water treatment plant
• Underground wind tunnel
• Utility vault
• Wheelhouse
• Wildlife tunnel
• Winze

Uncommon — only a minority of players land here.

• Aqueduct
• Borehole
• Burial vault
• Cave house
• Cellar dwelling
• Cesspit
• Cistern
• Crawl space
• Culvert
• Dugout
• Earth oven
• Earth-sheltered house
• Elevator shaft
• Escape tunnel
• Foundation
• Foxhole
• French drain
• Icehouse
• In-ground pool
• Kiva
• Manhole
• Military tunnel
• Mine tunnel
• Nuclear waste repository
• Oil well
• Ossuary
• Particle accelerator
• Pedestrian tunnel
• Pipeline
• Pit house
• Qanat
• Road tunnel
• Secret tunnel— Yeah, I forget the next couple of lines, but uh, then it goes
• Seed vault
• Septic tank
• Smuggling tunnel
• Storm drain
• Sub-basement
• Trapping pit
• Trench
• Tunnel network
• Underground base
• Underground city
• Underground laboratory
• Underground mall
• Underpass
• Underwater tunnel
• Utility tunnel
• Vault
• Ventilation shaft

A solid, middling pick shared with plenty of other players.

• Bomb shelter
• Catacombs
• Cellar
• Crypt
• Dungeon
• Fallout shelter
• Missile silo
• Railway tunnel
• Root cellar
• Storm cellar
• Subway station
• Tomb
• Underground garage
• Well
• Wine cellar

The most common answer on the board.

• Basement
• Bunker
• Mine
• Sewer
• Subway
• Tunnel