The Tower Subway is a tunnel beneath the River Thames in central London, between Tower Hill on the north side of the river and Vine Lane (off Tooley Street) on the south. In 1869 a 1,340 foot (410 m) circular tunnel was dug through the London clay using a wrought iron shield, a method that had been patented in 1864 by Peter W. Barlow. A 2 ft 6 in (762 mm) narrow gauge railway was laid in the tunnel and from August 1870 a cable-hauled wooden carriage conveyed passengers from one end to the other. This was uneconomic and the company went bankrupt by the end of the year. The tunnel was converted to pedestrian use and one million people a year crossed under the river, paying a toll of 1⁄2d. When the toll-free Tower Bridge opened in 1894 this caused a drop in income and the tunnel closed in 1898, after being sold to the London Hydraulic Power Company. The tunnel today is used for water mains.
Constructed 1869 beneath the River Thames near the Tower of London.
More information on the Wikipedia page is at [1] and the Subterranea Britannica page is [2].