NO ONE knows quite how many tunnels have been burrowed under London. Some of the city's many underground channels have been subject to the Official Secrets Act: the Kingsway tunnels were once used by a research arm of MI6, Britain's foreign-intelligence service. Others are so old they have fallen into disuse and been forgotten. Nonetheless, anyone walking through central London can safely assume that the busy streets around them are mirrored in criss-crossing tunnels below.
The world's first subterranean railway, the London Underground, opened in the city 150 years ago. Last month the first tunnel for Crossrail, an ambitious £15 billion ($24 billion) new railway project running across London, was completed. It lies 40 metres below the surface of the city, a wonder of engineering.
Why does London do so much tunnelling?
London has evolved tightly packed, with narrow streets and alleyways spreading out across it like arteries. Between 1801 and 1851 its population grew from around 960,000 to over 2.5m. Railway lines terminated at the edges of the city, as it was then defined, at Paddington and Victoria. As visitors and workers flocked to the city, its central roads became increasingly congested. Underground railways were built in 1863 to relieve the choked streets. Not everyone liked the early metro system, particularly as its trains relied on sooty steam engines."I had my first experience of Hades today," spluttered R.D. Blumenfeld, an American journalist, in 1887, "and if the real thing is to be like that I shall never do anything wrong."
Living in what was the wealthiest city in the world at the time, London's private investors could afford to sink money into the tunnelling schemes. While these tunnels were being built, London's sewerage system was also slowly being burrowed. Between 1859 and 1870 Joseph Bazalgette, an engineer, and his team built the system which is still in use today.
London's congestion created the need for tunnels and its booming economy provided the financing. But what made them feasible was the city's location. The clay on which most of the city sits provided an excellent tunnelling medium. It is soft enough to be excavated easily, but impermeable enough to stay dry. Once it is burrowed through it will not crumble. It has a "stand-up time," says Roger Bridge of the British Tunnelling Society; when the first Crossrail tunnel was being built, parts of the section could be bored out and then explored as the clay stayed in place. Harder rock would require more powerful machines or explosives to dig through it.
In contrast, cities such as Dublin are built on a mixture of sandstone and boulder clay. The boulders make it difficult to mine through, while the sand absorbs water, making it less sturdy. Before the development of better technology, such as pressure-balance machines, such conditions made tunnelling tricky. Indeed, parts of south London have fewer tunnels than the centre because fewer of its neighbourhoods are built on clay.
Other countries are catching up with London. The longest tunnel in the world was built in Switzerland in 2010; Bertha, the world's biggest boring machine, is currently whirring under Seattle. Hundreds of miles of tunnels are being built in China (the exact number is unknown; most are kept secret). Meanwhile, new tunnels in London will have to search hard to find a route. The Lee tunnel, part of a new "super-sewer" currently being built, is London's deepest ever. "As time goes on, we've been doing it for so long it is difficult to find a horizon to put a new tunnel through," says Mr Bridge. Below the surface, tunnels now have to deal with the above-ground congestion they were built to relieve.