April Air Force Base was an Air Force base in California that had been used as part of the Project to create an alien/human hybrid. Samantha Mulder and Jeffrey Spender were among the inhabitants. Fox Mulder, with help from Harold Piller discovered his sister's diary in the house on the base where she lived. The base had been decommissioned by the time Mulder visited. (TXF: "Closure") The real site of filming was San Bernardino International Airport, former Norton Air Force Base.