Photo: Myron Davis/TIME & LIFE Pictures

The construction of the Pentagon began, eerily enough, on September 11, 1941. To mark the 70th anniversary of the groundbreaking—and the 10th anniversary of the 9/11 attacks—LIFE.com is featuring a gallery of previously unpublished photos of the construction process.

Photo: Thomas D. McAvoy/TIME & LIFE Pictures

Photo: Myron Davis/TIME & LIFE Pictures

From LIFE (December 1942):
Both employees and visitors "resent the eight and two-fifths miles of barren corridors, the jammed ramps, the pile-up at entrances and exits, the parking and transportation problems, the six overcrowded cafeterias, the staggered working hours."

For many more previously unpublished photos of Pentagon construction, head over to LIFE.com.