A group of men in top hats and bowlers are standing at the entrance to a large metal structure which is marked 'Thames Iron Works and Ship-building Company Blackwall'. The Blackwall Tunnel under the River Thames in London was engineered by Alexander Binnie. It was built to provide a road link under the Thames between Greenwich and Poplar. When the tunnel opened fully in 1897 it was the longest underwater tunnel in the world, at 6,200 feet. Construction, using the tunneling shield technique pioneered by Marc Isambard Brunel in the building of his Thames Tunnel, was difficult. The tunnel had to be excavated through sand, and in places ran les than 6 feet beneath the river bed. A second prallel tunnel was opened in 1967 to ease traffic congestion.
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