Modifiers matching shelter:

• sturdy,
• temporary,
• open,
• informal,
• portable,
• humid,
• traditional,
• animal,
• inside,
• communal,
• nearby,
• fixed,
• larger,
• natural,
• safe,
• mobile,
• homeless,
• irregular,
• covered,
• indoor,
• small,
• private,
• recreational,
• permanent,
• erect,
• outdoor,
• makeshift,
• dark,
• alternative,
• up,
• artificial,
• protective,
• basic,
• damp,
• outside,
• under,
• major,
• smaller,
• warm,
• internal,
• separate,
• suitable,
• underground,

Simple categories matching underground:

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