Detroit-Windsor Tunnel Portal, Michigan

One of two vehicle crossings between Detroit, Michigan, USA and Windsor, Ontario, Canada, the Detroit-Windsor Tunnel goes under the Detroit River, instead of over it, like the much busier Ambassador Bridge. The tunnel portal is a spiral ramp downtown, next to General Motors headquarters, with a duty free shop, toll booth, and a US Port of Entry crammed into the space as well. Built in 1930, the tunnel is limited to two lanes of traffic, one in each direction, and no large trucks are permitted.  Still it sees around 13,000 vehicles a day. The international boundary is at the mid point of the mile-long tunnel, 75 feet below the surface of the river.  It is marked precisely with an International Boundary Commission plaque. This is said to be “the only international sub-aqueous vehicular tunnel in the world” (the Channel Tunnel between the UK and France is a train tunnel, not a vehicle tunnel).