Simple categories matching underground:

• area,
• system,
• excavation,
• world,
• structure,
• reservoir,
• facility,
• pipeline,
• construction,
• passage,
• equipment,
• obstruction,
• vegetable,
• resource,
• installation,
• part,
• tunnel,
• housing,
• improvement,
• hydrocarbon,
• room,
• location,
• enclosure,
• harvester,
• music,
• band,
• work,
• object,
• cistern,
• cave,
• works,
• formation,
• duct,
• pest,
• building,
• bunker,
• fortification,
• utility,
• feature,
• wealth,
• development,
• item,
• line,
• cavity,
• pipe,
• supply,
• activity,
• plant,
• fixture,
• topic,
• cable,
• obstacle,
• organ,
• place,
• property,
• liquid,
• space,
• cavern,
• relic,
• storage,
• service,
• conductor,
• group,
• shelter,
• life,
• tank,
• container,
• material,
• stem,
• fluid,
• aquifer,
• void,
• expanse,
• opening,
• conduit,
• treasure,

Members with property underground:

• basement_wall,
• cellar,
• basement,
• irrigation_facility,
• tar_tank,
• manhole,
• oil_tank,
• tunnel,
• roads,
• deep_foundation,
• twisted_machine,
• vault,
• riser,
• tank,
• kitchen,
• submarine_tunnel,
• tear_sail,
• acarid,
• missile,
• report,
• shaft,
• pipe,
• root,
• potato,
• building,
• sugar-beet_harvester,
• sewer_inlet,
• water,
• trench,
• underground_store,
• leach_line,
• pump,
• tunnel_excavation,
• bridge_pier,
• sewer,
• waste_repository,
• sanitary_sewer,
• sump-pump_drain,
• soil_improvement,
• mine_gallery,
• support_screen,
• fissure,
• void,
• utility,
• onion_harvester,
• gas_tube,
• metal_crate,
• earthquake,
• air_line,
• bulb,
• combine,
• limestone_cavity,
• workshop,
• underground_city,
• mineral,
• wire_worm,
• tunnel_system,
• pedestal,
• art,
• postal_service,
• water_pipeline,
• seepage_pit,
• potato_cob,
• waste_oil,
• construction,
• archaeological_feature,
• drain_tile,
• pull_box,
• disused_oil,
• child_prostitution,
• electrical_conduit,
• fuel_tank,
• system,
• sedimentary_wedge,
• fur_industry,
• damp_cellar,
• energy_storage,
• sand,
• depleted_oil,
• resource,
• rock_burst,
• chalk,
• flooded_bunker,
• sewer_connection,
• rail_tunnel,
• electric_vault,
• abandoned_warehouse,
• chasm,
• yard_light,
• stratum_control,
• root_tuber,
• subway_line,
• underground_installation,
• water_service,
• town-gas_pipeline,
• transportation_tunnel,
• connection,
• old_foundation,
• underground_passage,
• rock_fissure,
• loading_station,
• underground_cable,
• aphid,
• drainage_pipe,
• depth,
• sinkhole,
• tv_cable,
• size_conductivity,
• compressor,
• insect,
• gas_outburst,
• experiment_hall,
• sink_hole,
• beam,
• trench_box,
• sewer_pipe,
• utility_duct,
• igloo_cave,
• pamphlet_distribution,
• bunker,
• white_grub,
• traffic_tunnel,
• spade,
• cable-television_network,
• scoop_tram,
• leak,
• isolation_transformer,
• cave,
• barrels,
• truffle,
• stone,
• sewerage_tunnel,
• unitization,
• steel_penstock,
• tunnel_work,
• yakuza,
• match,
• cali_agent,
• bud,
• tower,
• conduit,
• corn,
• gambling,
• sprinkler,
• document,
• pipeline_construction,
• pipe_line,
• oil_field,
• tube_station,
• unexploded_ordnance,
• office_space,
• truck,
• foundation,
• secret_abbey,
• rolling_moth,
• shield_construction,
• stairs,
• drain,
• brine,
• dome,
• hardware,
• gas,
• subsurface_structure,
• pipework,
• sewer_building,
• ore_deposit,
• inclined_shaft,
• ghetto_newspaper,
• underpass,
• silver,
• carrot,
• tuber,
• city_sewer,
• public-utility_conduit,
• explosion,
• ancient_building,
• muslim_brotherhood,
• switch,
• underground_engineering,
• street_cabinet,
• television_cable,
• driveway,
• drift_outfitting,
• bass,
• tunnel_liner,
• line,
• utility_line,
• garage,
• utility_service,
• beetroot,
• subway,
• sewerage,
• stream,
• fresh_water,
• salt_dome,
• utility_meter,
• team,
• boring_machine,
• cavern_tunnel,
• los-alamos_base,
• liquidation,
• gas_reservoir,
• gas_pipe,
• natural_hazard,
• underground_structure,
• graves,
• main,
• cavity_structure,
• sewage_pipe,
• valve_pit,
• storage_facility,
• telephone,
• water-supply_pipeline,
• meter_cabinet,
• international_answer,
• plastic_pipe,
• scraper_winch,
• cult,
• wind-turbine_foundation,
• railway,
• surface,
• box,
• archaeological_site,
• substation,
• rock,
• access,
• cistern,
• silver_storeroom,
• telecommunication_facility,
• air-raid_shelter,
• mhz,
• synagogue,
• submersible_pump,
• communication_line,
• basilica_cistern,
• water-supply_pipe,
• bridge_culvert,
• road_tunnel,
• salt_cavity,
• nematode,
• intake_structure,
• service_box,
• store,
• sabotage,
• injector_tunnel,
• hydrant,
• aesop_rock,
• tube,
• clark_cavern,
• underground_repository,
• medium,
• existing_tank,
• telecommunication_line,
• detention_vault,
• water-meter_box,
• chamber,
• storm_sewer,
• dream_syndicate,
• bridge_abutment,
• icehouse,
• capacitor,
• underground_market,
• invisible_fencing,
• foundation_wall,
• disused_tunnel,
• retail_space,
• turnip,
• lava_tube,
• klystron_tunnel,
• catacomb,
• trench_wall,
• pole,
• access_tunnel,
• sand_layer,
• deep_excavation,
• storm_drain,
• rutabaga,
• avoidance,
• geological_structure,
• old_oil,
• plumbing,
• cutworm,
• mining_machine,
• invisible_fence,
• mt_shasta,
• paving,
• depot,
• gravel,
• transport,
• service_tunnel,
• reef,
• pvc_pipe,
• termite,
• foundation_element,
• wire,
• light,
• waste_package,
• gas_pipeline,
• pocket,
• terminal_box,
• mining,
• church,
• wet_basement,
• underground-railway_tunnel,
• stern_gang,
• pipeline,
• underwater_pipe,
• subway_tunnel,
• water_pocket,
• operations,
• gang,
• shield_works,
• sewer_pipeline,
• building_drain,
• command_bunker,
• command_center,
• sewage_conduit,
• transformer,
• excavation,
• subterranean_city,
• anode_bed,
• hand_hole,
• cutout,
• valve_box,
• house_foundation,
• concrete,
• subterranean_room,
• anchor,
• water_distribution,
• beetle,
• salt_cavern,
• trench_system,
• grub,
• similar_works,
• circle,
• pit,
• box_culvert,
• drainage_structure,
• meter,
• telephone_conduit,
• hobby_room,
• hi,
• duct_structure,
• parking_garage,
• electric,
• plaza_deck,
• culvert,
• road,
• telephone_cable,
• underground_facility,
• arch,
• virus_nine,
• septic_system,
• former_dungeon,
• curb_inlet,
• pillbox,
• adit,
• mining_shaft,
• corm,
• corruption,
• plow,
• potato_harvester,
• rootstock,
• shaft_sinking,
• computer,
• oil,
• power,
• vertical_shaft,
• test,
• basement_cellar,
• dumper_pit,
• soft_cell,
• trinity_aquifer,
• platform,
• mono_pump,
• valve,
• ginger,
• level,
• haganah,
• cavity,
• hole,
• electrical_cable,
• conveyor-belt_system,
• utility_vault,
• soil_acid,
• telecommunication_vault,
• liquid,
• utility_pipe,
• metro_system,
• electrical_installation,
• hives,
• brickwork,
• stone_floor,
• telecommunication_conduit,
• sewerage_pipe,
• oil_reservoir,
• silo,
• station,
• gallery,
• stem,
• rodent,
• mine_shaft,
• concrete_foundation,
• wind,
• sink,
• raid,
• repository,
• white_brotherhood,
• depleted_reservoir,
• particle_separator,
• gas_field,
• new_york,
• ditch,
• sewage,
• electric_cable,
• shaft_furniture,
• design,
• electric_light,
• ohio_seven,
• pilot,
• radish,
• service,
• side,
• wire_cable,
• junction_box,
• saltwater_aquifer,
• salt_cave,
• track,
• telecommunication_cable,
• water_workplace,
• passage,
• prostitution,
• yam,
• petroleum_storage,
• amplifier,
• drift,
• quarry,
• dam,
• conductor,
• mine,
• hell_angle,
• breaker,
• service_pole,
• steel_tank,
• tunnel_building,
• active,
• utility_tunnel,
• spring,
• fungus,
• mining_excavation,
• irrigation_structure,
• cable,
• gold,
• ore_body,
• accelerator_tunnel,
• shiny_treasure,
• drift_area,
• global_hell,
• horizontal_boring,
• water_issue,
• duct,
• complex,
• drainage_culvert,
• missing_foundation,
• underground_sprinkler,
• detective_area,
• real-time_information,
• tomb,
• rail,
• mole,
• civil_defense,
• command_facility,
• elf,
• gopher,
• ventilation,
• storage-tank_due,
• discharge_line,
• electric_line,
• rhizome,
• abandoned_line,
• inlet,
• fuel_pipeline,
• beet,
• water_dome,
• drum,
• ruin,
• septic_tank,
• large_building,
• nuclear_waste,
• long_rhizome,
• parsnip,
• pier,
• concrete_block,
• sump,
• raven_pit,
• footing,
• building_foundation,
• onion,
• crime,
• sewer_work,
• communication_cable,
• structure,
• cavern,
• flooded_vault,
• irrigation_service,
• disused_mine,
• telecommunication,
• natural_gas,