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.
The same method of construction was used in 1890 to dig the tunnels of the City and South London Railway, the first of London's "Tube" railways.In 1864 Peter Barlow patented a method of tunnelling using a circular wrought iron or steel shield and filling the gap between the tunnel lining with lime or cement to prevent settling of the ground above. He published a pamphlet in 1867 suggesting a network of tunnels with cars carrying up to 12 people. In 1868 authority was obtained for a tunnel under the Thames between Great Tower Hill and Pickle Herring Stairs near Vine Street (now Vine Lane), 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.
Work began in February 1869 with the boring of entrance shafts, 60 feet (18 m) deep on the north bank and 50 feet (15 m) deep on the south bank. The tunnelling itself started in April and, using the circular shield, a tunnel 1,340 feet (410 m) long was dug with a diameter of 6 feet 7 3⁄4 inches (2.026 m), a maximum of 66 feet (20 m) below the high-water level. This was bored through a stable layer of the London clay that lay 22 feet (6.7 m) below the river bed, below the soft alluvial deposits that had plagued the construction by Brunel of the earlier Thames Tunnel. This, combined with the simpler nature of the project – the excavation face was only one twentieth that of the Thames Tunnel – enabled faster progress. Screw jacks drove the shield forward at a rate of 37 feet 4 inches (11.38 m) each week. The under-river section was dug in fourteen weeks and the tunnel completed in December.
The entrance shafts were fitted with steam-powered lifts for passengers. The tunnel was laid with 2 ft 6 in (762 mm) gauge railway track and a single car, carrying a maximum of 12 passengers, cable-hauled by two 4 horsepower (3.0 kW) stationary steam engines, one on each side of the river. The underground railway opened on 2 August 1870 charging 2d for first class and 1d for second class, first class ticket holders merely having priority when boarding. However, the system was unreliable and uneconomic, and closed that December after the company went bankrupt.
The rail car and steam engines were removed, gas lights installed and the passenger lifts replaced with spiral staircases. The tunnel opened to pedestrians at a toll of 1⁄2d and became a popular way to cross the river, averaging 20,000 people a week (a million a year). Its main users were described as "the working classes who were formerly entirely dependent on the ferries". In September 1888 the subway briefly achieved notoriety after a man with a knife was seen in the tunnel at the time when Jack the Ripper was committing murders in nearby Whitechapel.
In his Dictionary of London, Charles Dickens, Jr. commented on the smallness of the tunnel: "there is not much head-room left, and it is not advisable for any but the very briefest of Her Majesty's lieges to attempt the passage in high-heeled boots, or with a hat to which he attaches any particular value."
The Italian writer Edmondo De Amicis (1846–1908) gave a description of a passage through the subway in his Jottings about London:
As I was thinking of these things I disappeared from the world indeed, going down a lighted spiral staircase which buries itself in the earth on the right bank of the Thames, opposite the Tower. I went down and down between two dingy walls until I found myself at the round opening of the gigantic iron tube, which seems to undulate like a great intestine in the enormous belly of the river. The inside of this tube presents the appearance of a subterranean corridor, of which the end is invisible. It is lighted by a row of lights as far as you can see, which shed a veiled light, like sepulchral lamps; the atmosphere is foggy; you go along considerable stretches without meeting a soul; the walls sweat like those of an aqueduct; the floor moves under your feet like the deck of a vessel; the steps and voices of the people coming the other way give forth a cavernous sound, and are heard before you see the people, and they at a distance seem like great shadows; there is, in short, a sort of something mysterious, which without alarming causes in your heart a vague sense of disquiet. When then you have reached the middle and no longer see the end in either direction, and feel the silence of a catacomb, and know not how much farther you must go, and reflect that in the water beneath, in the obscure depths of the river, is where suicides meet death, and that over your head vessels are passing, and that if a crack should open in the wall you would not even have the time to recommend your soul to God, in that moment how lovely seems the sun!
I believe I had come a good part of a mile when I reached the opposite opening on the left bank of the Thames; I went up a staircase, the mate of the other, and came out in front of the Tower of London.
In 1894 the toll-free Tower Bridge opened a few hundred yards downriver, causing a drop in the subway's income. In 1897, Parliament passed an Act authorising the sale of the tunnel to the London Hydraulic Power Company (LHPC) for £3,000, and the subway closed to pedestrian traffic in 1898.
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 (1.2 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.
While it is no longer used for hydraulic tubes, the tunnel still carries water mains. The hydraulic tubes, once a major source of power in the centre of London, have since been replaced by telecommunication cables.
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 the 1920s 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. It is located just behind the Unicorn Theatre on Tooley Street, but there is no plaque to mark the site.