This was once the entrance to the Tower Subway, built in 1870 to provide a cable-car service under the Thames to Southwark. It was completed by J H Greathead in just 10 months at a fraction of the time and cost of the Brunel's earlier and ill-fated Thames Tunnel between Wapping and Rotherhithe which now forms part of the East London Underground line.
Work on the Tower Subway was hastened by the use of the 'Greathead Shield, which proved so successful that it was the forerunner of the methods used to build London's Tube system at the end of the nineteenth century.
Greathead, one of London's unsung heroes, was the engineer in charge of the King William Street to Stockwell line (now the City branch of the Northern Line), opened in 1890 as the world's first deep-level railway.
Sadly, Tower Subway was never very successful. The cable-cars were soon taken out of the tunnel, which was seven feet in diameter and more than 440 yards long, and it became a pedestrian passageway. In 1894 the subway was made redundant by the walkway incorporated into the new Tower Bridge. Today all the subway contains are water mains for the Thames Water Authority.