Assertion: ufo | /r/HasProperty | true unknowns
Facets:
Source sentences:
text: Two of the country's best military photo laboratories, the Air Force lab at Wright Field and the Navy's lab at Anacostia, Maryland, had spent many hours trying to prove that the UFO's were balloons, airplanes, or stray light reflections, but they failed - the UFO's were true unknowns.