Mining operations at the Greenspring Quarry began in the mid 1800s, and its stone was used to build railroad beds for the transportation of supplies during the Civil War. In later years, its mined materials were used in construction of the Baltimore Beltway and other local developments, including Harbor Place, Fort McHenry Tunnel, Johns Hopkins Hospital, National Aquarium, and the stadiums at Camden Yards. When excavation ended in December 1999, an estimated 35 million tons of rock had been removed, and the bottom of the quarry extended to a depth of more than 500 feet, 50 feet below sea level.