There are several underground tunnels running beneath downtown Austin that serve as aqueducts for active artesian springs. The best-known of these runs N/S along Nueces Street from 4th to 19th street. The entrance is just across the street from La Zona Rosa down along the banks of Shoal Creek. It was closed up about ten years ago. It's an active branch of Shoal Creek, (shown on old maps as "Little Shoal Creek) that used to be on the surface but was covered over in the 1920s when much of the southwest side of downtown was backfilled and leveled-off to accomodate the construction of new city facilities. The arched walls/ceiling are constructed of beautiful hand-hewn limestone, and the old limestone bridge pilings at each cross-street were incorporated into the walls. There is another one that runs from an underground spring at 9th street south along Congress Avenue until it dumps into Town Lake. I don;t think it's at all accessible to the public either. All that fresh spring water just travels on down to Bastrop and beyond.
Lotsa cool stuff still to be found in Lost Austin...