Project Blue Book Case #2542. On April 15, 1953, an unidentified object was observed over Tucson, Arizona — the sixth Blue Book unknown from this desert city near Davis-Monthan AFB. With six cases spanning 1949 to 1953, Tucson held the uncontested record for the most Blue Book unknowns from any medium-sized American city.
The persistence of unknowns over Tucson was extraordinary. Six cases in four years, across three different investigation eras (Grudge, early Blue Book, and post-Robertson Panel), suggested a genuine recurring phenomenon rather than a wave-dependent pattern.
The case was classified "Unknown."