Modifiers matching formation:

• geologic,
• crystalline,
• geological,
• porous,
• political,
• pathologic,
• impermeable,
• social,
• weak,
• tall,
• solid,
• underwater,
• natural,
• smaller,
• stable,
• nearby,
• monolithic,
• underground,
• morbid,
• unusual,
• structural,
• historical,
• sacred,
• geometric,
• strong,
• organic,
• discrete,
• soft,
• medium,
• polygonal,
• hard,
• standard,
• conical,
• up,
• special,
• vertical,
• softer,
• dense,
• spatial,
• individual,
• pointed,
• personal,
• geophysical,
• past,
• defensive,
• traditional,
• volcanic,
• internal,
• galactic,
• glacial,
• rocky,
• artistic,
• particular,
• formal,
• exterior,
• salt,
• elastic,
• active,
• permeable,
• complex,
• mental,
• subsurface,
• christian,
• deep,
• molecular,
• earthen,
• larger,
• plastic,
• small,
• sedimentary,
• military,
• cultural,
• harder,
• quaternary,
• virtual,
• impersonal,
• strange,
• massive,
• ancient,
• major,
• acid,
• abrasive,
• siliceous,
• driving,
• coastal,
• hollow,
• pedestrian,
• ideological,
• organized,
• typical,
• beautiful,
• symbolic,
• impervious,
• unconsolidated,
• territorial,
• interesting,
• surrounding,
• diverse,
• offer,
• undesirable,
• popular,
• cambrian,
• calcareous,
• homogeneous,
• sticky,
• intricate,
• geographic,
• tropical,
• key,
• uniform,
• igneous,
• thick,
• regular,
• conceptual,
• unstable,
• geographical,
• unique,
• moving,
• large,
• national,
• separate,
• common,
• spiritual,
• rare,
• impressive,
• star,

Simple categories matching underground:

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