The Carroll Water Treatment Plant is a new site in the chain of water facilities for Boston’s water supply. It is connected to the new Metro West Tunnel, and the Hultman Aqueduct. For more than 17 miles, and more than 50 years, the Hultman Aqueduct carried nearly all of metropolitan Boston's water, a crucial link in the only water system of any major city in the country that didn't have a back up. By the late 1990s, the surface level pipeline, built in 1940, and snaking through suburban Boston as an earth covered mound, was crumbling, and had over 40 known leaks. A second tunnel, the Metro West Tunnel, deep underground, went online in 2003, after several years of construction. Until the Hultman is repaired, a process that is taking years, the Metro West Tunnel is now the non-redundant water supply tunnel.