The Ted Williams TunnelThe . 75-mile underwater part of the 1.6 mile tunnel used a dozen steel tube sections, each longer than a football field. These were placed into a trench on the Boston Harbor floor and were then connected.
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First, workers dredged a 50-foot trench along the floor of Boston Harbor. Then, 12 giant steel tubes, each 325 feet long and already containing roads, were dropped into the water. Once the tubes had been connected on the harbor floor, the tunnel was buried in a 5-foot protective layer of rock.
The Eiksund road tunnel in Norway connects the Norwegian mainland with Hareidlandet island. The tunnel is 7,765 metres (25,476 feet) long and achieves a depth below sea level of 287 metres (942 feet).
Collapse of the tunnel isn't what would make it impassible first. The tunnel itself might well last a century or so, but if there's no electric power for as little as a few weeks the tunnel will be closed by water seepage that can't be pumped out.
The answer to this may surprise you. The tunnel does leak, but it is designed to. Seawater from the rocks above the tunnel drips through and is then pumped away.
The tunnel is actually designed to leak. As Eurotunnel explain on their website, 'seawater from the rocks above the tunnel drips through and is then pumped away.