Detroit-Windsor Tunnel, Detroit

This mile-long tunnel, constructed in 1930 under the Detroit River, is the first vehicular tunnel ever built linking two nations. Heavily used by commuters and travelers, it is one of the busiest border crossings between the United States and Canada, handling nine million vehicles a year, 95 percent of them cars. Its U.S. entrance is right next to the Renaissance Center, and its Canadian outlet is in the middle of Windsor's downtown. Jointly owned by the cities of Windsor and Detroit, it has an elaborate ventilation system that keeps the air clean. It was renovated in the mid-1990s. On the Canadian side, a Duty Free Shoppe offers tax-free purchases.