Ted Williams Tunnel, Massachusetts

The Ted Williams Tunnel connects Interstate 93 to Boston's Logan Airport, traveling under Boston Harbor. The 1.6-mile tunnel is of the immersed tube type (as opposed to dug tunnels), where pre-made tube sections are sunk into place and fastened together. It was part of the Big Dig, the nation's most expensive urban highway program in recent history, that transformed Boston's waterfront. The 20 year long project was centered around Interstate 93's congested path through the city, and the relocation of the roads connected to it. I-93 itself was relocated below ground, removing the narrow and obtrusive elevated Central Artery freeway through some of Boston's most valuable real estate. By the time the last bond is paid off, the current estimate for the total cost of the Big Dig (when interest is included) is more than $21 billion.