Simple categories matching unknown:

• object,
• event,
• content,
• threat,
• actor,
• program,
• agent,
• substance,
• entity,
• reason,
• infection,
• animal,
• process,
• thing,
• disease,
• instrument,
• information,
• dimension,
• knowledge,

Members with property unknown:

• space_debris,
• credit_derivative,
• length,
• Brandon_Routh,
• worm,
• date-rape_drug,
• animal,
• unknown,
• cell_differentiation,
• liquid,
• climatic_change,
• organized_crime,
• comet,
• asteroid,
• virus,
• smallpox,
• drug,
• tobacco,
• vehicle_identification,
• medicine,
• diphtheria,
• measles,
• tuberculosis,
• meaning,
• economic_factor,
• death,
• return_stream,
• alloy,
• old_bottle,
• existence,
• dysentery,
• depth,
• household_cleaners,
• hiv,
• flagpole,
• temperature,
• construction,
• dark_energy,
• building,
• ufo,
• heretic_empire,
• research,
• atlantis,
• scleroderma,
• philosophical_matter,
• unknown_event,
• new_moon,
• football_match,
• product,
• mountain,
• foreigner,
• ghost,
• debris,
• fertilizer,
• imperfect_sphere,
• steep_terrain,
• star_formation,
• ph_value,
• avian_flu,
• malign_demon,
• tree,
• data_collection,
• medical_emergency,
• marimba,
• vehicle,
• banjo,
• hepatitis,
• fire_debris,
• paint,
• pesticide,
• creation,
• pcp,
• traffic_flow,
• cancer,
• metal,
• ring_system,
• future_storm,
• eclipse,
• plant_delay,
• pleurisy,
• chemical,
• paper_debris,
• wind,
• metabolite,
• economic_reason,
• protein,
• bitumen,
• prophylactic_antibiotic,
• sleigh,
• cd,
• new_planet,
• angle,
• entrepreneurial_enterprise,
• loch-ness_monster,
• cell,
• street_silicone,
• lipid,
• space,
• stroke,
• sars,
• immigrant,
• small_shop,
• fishpond,
• observation,
• violin,
• tube_worm,
• earthquake,
• paraffin,
• carbohydrate,
• small_business,
• clothing_fiber,
• gas,
• methamphetamine,
• explosive,
• dna_fragmentation,
• gamma-hydroxybutyrate_ghb,
• dust,
• cold,
• furosemide,
• heights,
• image,
• star,
• poison,
• fact,
• indictment,
• soul,
• width,
• novel_pathogen,
• protease_activation,
• white_powder,
• product_property,
• pattern,
• adulterated_silicone,
• ninja,
• fundamentalist,
• aurora,
• trojan,
• rule,
• chemical-terrorism_agent,
• importance_factor,
• audio,
• product_condition,
• small_boat,
• amazon,
• light,
• powder,
• strong_indication,
• replication,
• risk,
• individual_infection,
• writings,
• controlled_experiment,
• illicit_drug,
• vibraphone,
• god,