An unmarked airplane was spotted in Fresno this past weekend circling the city’s airport, at least the third such incident in a string of unusual appearances in the state of military-level aircraft. The Navy Boeing E-6B Mercury, nicknamed the “Doomsday Plane,” flew about 30 minutes Sunday in an unusually low level. It was not clear what the aircraft was doing. That type of plane typically provides airborne headquarters and communications for the U.S. Navy. The aircraft serves as a nuclear