Modifiers matching obstacle:

• nearby,
• natural,
• world,
• wooden,
• physical,
• artificial,
• private,
• high,
• everyday,
• fixed,
• stationary,
• dynamic,
• soft,
• over,
• special,
• large,
• dangerous,
• advanced,
• common,
• visual,
• psychological,
• linear,
• environmental,
• small,
• solid,
• urban,
• typical,
• structural,
• distant,
• unmovable,
• single,
• passive,
• permanent,
• penetrating,
• coming,
• extensive,
• additional,
• simple,
• outdoor,
• thick,
• geological,
• dense,
• external,
• concrete,
• difficult,
• interfering,
• tall,
• overhead,
• rigid,
• practical,
• mechanical,
• rural,
• unusual,
• porous,
• institutional,
• elevated,
• rectangular,
• multiple,
• opaque,
• local,
• vertical,
• tricky,
• adjacent,
• working,
• impenetrable,
• hazardous,
• movable,
• creative,
• removable,
• clear,
• narrow,
• size,
• moving,
• static,
• geographic,
• underground,
• architectural,

Simple categories matching underground:

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