It is a narrow two-lane road tunnel dating back to 1908. Its sharp curves served two purposes: avoiding the local docks on each side of the river, and preventing horses from seeing daylight at the end of the tunnel too early which might make them bolt for the exit.
It is not to be confused with an earlier and much more historic tunnel, designed and built under the supervision of Marc Isambard Brunel and his son Isambard Kingdom Brunel, originally as the Thames foot tunnel, but now carrying the East London Line, part of the London Underground.