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Tunnel beneath the River Thames in Central London, England

The Tower Subway is a tunnel beneath the River Thames in central London, between Tower Hill on the north bank of the river and Vine Lane (off Tooley Street) on the south. In 1869 a 1340 feet (408 m) circular tunnel was dug through the London clay using a cast iron circular shield independently invented and built by James Henry Greathead, similar to an idea that had been not received a patent in 1864, nor built by Peter W. Barlow. A 2ft6in 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 a ha'penny. The opening of the toll free Tower Bridge nearby in 1894 caused a drop in income and the tunnel closed in 1898, after being sold to the London Hydraulic Power Company. Today the tunnel is used for water mains and telecommunications cables. The same shield method of construction was used in 1890 to dig the tunnels of the City and South London Railway, the first of London's electrified "Tube" railways and the first underground electrified railway in the world.

In 1864 Peter Barlow patented a method of tunnelling using a circular cast iron shield and to fill the gap between the tunnel lining and wall with lime or cement to prevent settling of the surrounding ground. Unfortunately, Barlow failed to explain how he intended to fill such gaps between shield and tunnel wall with grout. He published a pamphlet in 1867 suggesting a network of tunnels with cars carrying up to twelve people. but there was a delay finding a contractor due to recent experiences with the Thames Tunnel until his former pupil James Henry Greathead tendered for £9,400. in New York as the editor of The Scientific American, and may well have heard of Tower Subway developments in London. Work began in February 1869 with the boring of entrance shafts, The under river section was dug in fourteen weeks and the tunnel completed in December 1869.

The entrance shafts were fitted with steam powered lifts for passengers. The tunnel was laid with 2ft6in gauge railway track and a single car, carrying a maximum of 12 passengers, cable hauled by two stationary steam engines, one on each side of the river. The tunnel was completed by February 1870, and a press launch was held the following April. The underground railway opened for public use on 2 August 1870 charging 2d for first class and 1d for second class, first class ticket holders merely having priority for the lifts and when boarding. However, the system was unreliable and uneconomic. The company went into receivership in November 1870, and the railway closed on 7 December 1870, four months after opening.

After its closure, the tunnel gained a new purpose as a route for hydraulic power mains operated by the LHPC and for water mains. It was damaged during the Second World War when a German bomb fell in the river near Tower Pier in December 1940, and exploded on the river bed very close to the tunnel's roof. The shock of the blast compressed the tunnel radially, reducing its diameter to 4 feet (1 m) at the point of impact, but the tunnel's lining was not penetrated. During the course of repair work, it was found that – apart from the bomb damage – the tunnel had survived seventy years of use in excellent condition. The hydraulic tubes, once a major source of power in the centre of London, have since been replaced by fibre optic telecommunications links. A small round entrance building survives at Tower Hill near the Tower of London's ticket office, a short distance to the west of the main entrance to the Tower. This is not the original entrance but was built in 1926 by the London Hydraulic Power Company, with a ring of lettering giving the original date of construction and naming the LHPC. The entrance on the south bank of the Thames was demolished in the 1990s, and a new one has been built in its place. A video inside the tunnel from the current owners, Vodafone, was released in February 2023.