In the mid-1970s, a third subterranean shopping venue was built that connected into existing Crystal Plaza Shops and Crystal Mall Shops concourses. The first tenants in the Crystal Underground shopping village commenced operation in June 1976. An official dedication was held in October.

Graphic from the Charles E. Smith Companies

Crystal Underground storefronts were done in an old timey, 1920s motif. 

Drawing from the Charles E. Smith Companies

The DC Metro initiated service on a 12-route-mile stretch of subway in July 1977. Crystal City Station, seen here, was one of eighteen stops on the initial Blue Line route. In April 1983, the Metro Yellow Line was dedicated, which also serviced the Crystal City stop.
Photo from Wikipedia / Ben Schumin

A skylighted court area in the subterranean Crystal Plaza Shops. This snapshot was taken in 1986.

A circa-1986 street view of the Crystal Mall superblock. The Crystal City Marriott hotel is seen on the left.

Crystal Underground had been given a face lift by the time this 1990s snapshot was taken.

Photo from https://www.arlnow.com/ "JBGS"