||This article needs attention from an expert in London. (November 2008)|
These have served a number of purposes:
Water and waste 
Since its foundation, the Thames has been at the heart of London. Many tributaries flow into it and over time these have changed from sources of water to open sewers and sources of disease.
As the city developed from a cluster of villages, many of the existing rivers were buried or canalized: see subterranean rivers of London.
The rivers failed to carry the sewage of the growing metropolis. The resulting health crisis led to the creation in the late nineteenth century of the London sewerage system, designed by Joseph Bazalgette, one of the first modern sewer systems in the world.
The Thames Water Ring Main is a notable modern piece of large-scale water supply infrastructure, comprising 80 km of wide-bore water-carrying tunnels.
The London Underground was the first underground railway in the world, and remains one of the most extensive. Its construction began in 1860 with the 3.7-mile (6.0 km) Metropolitan Railway from Farringdon to Paddington. It was opened in 1863, having caused much disruption by the use of "cut and cover" techniques, which involved digging large trenches along the course of existing roads, and then constructing a roof over the excavation to reinstate the road surface. Tube railways, which caused little disruption because they were constructed by boring a tunnel, arrived in 1890, with the opening of the City and South London Railway, a 3.5-mile (5.6 km) line from Stockwell to King William Street. It was planned as a cable hauled railway, but the advent of electric traction resulted in a simpler solution, and the change was made before the cable system was built. It thus became the world's first electric tube railway. Although the system comprises 249 miles (401 km) of track, only about 45 per cent is actually below ground.
Numerous tunnels underneath the River Thames have been created, ranging from foot-tunnels to road tunnels and the tunnels of the Underground. The first of these, the Thames Tunnel, designed by Marc Brunel, was the first tunnel known to have been successfully constructed underneath a navigable river. It ran for 1,200 yards (1,100 m) from Rotherhithe to Wapping, and was opened in 1843. It was used as a pedestrian subway, as the finance was not available to allow the company to build the intended access ramps for horse-drawn traffic, and was later used by the East London branch of the Metropolitan Railway from Shoreditch to New Cross.
Several railway stations have cavernous vaults and tunnels running beneath them, often disused, or reopened with a new purpose. Examples include The Old Vic Tunnels, beneath London Waterloo station, and the vaults beneath London Bridge station, formerly utilised by the theatre company Shunt.
Many underground military citadels have been built under London. Few are acknowledged to exist and even fewer open to the public. One exception is the famous, and now very popular tourist destination, Cabinet War Rooms, used by Winston Churchill during the Second World War.
During the Second World War, parts of the Underground were converted into air-raid shelters known as the deep-level shelters. Some of these were converted for military and civil defence use, such as the now-disused Kingsway telephone exchange.
Other civil defence centres in London are wholly or partly underground, mostly as a legacy of the Cold War. Many other subterranean facilities exist within the centre of government in Whitehall, many linked by underground tunnels.
In 1980 a journalist for the New Statesman revealed the existence of secret tunnels linking government buildings, which he claimed would be used in the event of a national emergency. It is believed these tunnels link to Buckingham Palace.
London, like most other major cities, also has extensive underground infrastructure for electricity distribution, natural gas supply, water supply, and telecommunications, including the BT copper local loops and optical fibre from numerous suppliers.
Starting in 1861, Victorian engineers built several miles of purpose-built subways large enough to walk through, for running gas, electricity, water and hydraulic power pipes through. These works removed the inconvenience of continually excavating highways to allow access to underground utilities.
Some underground structures continue to exist in London even though they are no longer in use
- The London Hydraulic Power Company, set up in 1883, installed a hydraulic power network of high-pressure cast iron water mains under London. These were bought by Mercury Communications for use as telecommunications ducts.
- Some sections of the London Pneumatic Despatch Company tunnels linking the General Post Office and Euston Railway station.
- An extensive private underground railway, the London Post Office Railway, was constructed by the Post Office, but is now no longer used.
- There are some closed London Underground stations which are no longer accessible to the public.
See also 
- List of former and unopened London Underground stations
- Military citadels under London
- London deep-level shelters
- Tunnels underneath the River Thames
- London sewerage system
- Catacombs of London
- Subterranean rivers of London
- Neverwhere, a story set in a fantasy underground London
Individual sites of interest:
- Kingsway tramway subway
- Criterion Theatre
- Tower Subway
- King William Street tube station
- Holborn Viaduct Low Level Station
- Oxgate Admiralty Citadel
- Bishopsgate railway station
- Northern Outfall Sewer
- Southern Outfall Sewer
- Great Conduit
- John Glover, (1996), London's Underground, 8th Ed., Ian Allen Publishing, ISBN 0-7110-2416-2
- Charles E. Lee, (1967), Sixty Years of the Northern, London Transport
- Transport For London, Key Facts, accessed 16 January 2009
- Laurie, Peter (1979). Beneath the City Streets. Panther. pp. 183–211. ISBN 978-0-586-05055-2.
- Duncan Campbell "A Christmas Party for the Moles" New Statesman 19–26 December 1980
- Antony Clayton (2000) Subterranean City ISBN 0-948667-69-9
- Emmerson, A. and Beard, T. (2004) London's Secret Tubes, Capital Transport Publishing, ISBN 1-85414-283-6.
- Trench, R. and Hillman, E. (1993) London Under London: A subterranean guide, second revised edition, London: John Murray, ISBN 0-7195-5288-5.
- Campbell, Duncan (24 November 1983) War Plan UK. Granada, UK. ISBN 0-586-08479-7 & ISBN 978-0-586-08479-3.