Travel to 23-24 Leinster Gardens, West London, by road and you are unlikely to spot anything out of the ordinary - although your attention may be drawn by the two rather grand five- storey townhouses located at the address. But no amount of knocking on the front doors will raise the residents. Because there are none.
The once-grand houses were demolished in the 1860s to make way for part of the newly constructed District line and the facade - rebuilt to match the surrounding buildings - shields an open section of the track where the original trains could, literally, let off steam.
The London Underground, as we now know it, is 150 years old. The world's first underground rail line, the Metropolitan Railway, opened to great acclaim in 1863 with what would now be considered a meagre stretch of four-mile (6km) track, running from Paddington to Farringdon.
Nowadays such engineering is taken largely for granted, superseded by projects such as the Channel Tunnel and the Dahuofang Water Tunnel in China, one of the longest tunnels for water distribution in the world. But in Victorian times the complex system of tunnels was regarded as a modern marvel. And the construction still has historical lessons for engineering students today.
It may surprise them to learn that, despite its title, only 45 per cent of the London Underground runs through tunnels. And the history and construction of those tunnels is fascinating. In fact, many began life as open-cast excavations. The Metropolitan, District and Hammersmith and City lines, as well as part of the former East London line, for example, were not built by tunnelling underneath the streets of London but created using the "cut-and-cover" method. The streets were excavated and, at the bottom of the trenches, tracks were laid. A brick-lined tunnel, complete with curved roof, was then constructed over the tracks, the excavation was backfilled and the road restored on top. Ironically, the underground railway was built to provide a solution to major congestion in the streets but the construction added to those very problems; the building of other "cut-and-cover" lines was abandoned by the end of the 19th century.
Today's techniques have come a long way from those employed by miners hundreds of years ago when rock falls were common. Digging safe tunnels requires a knowledge and understanding of the material you are excavating as well as superb engineering skills. Over the decades engineers and geologists devised and perfected methods of tunnel-making that allowed the incredible expansion of the London Underground. In some cases, existing tunnels were reused, remodelled and brought into the modern network.
Water over the bridge
A key name in any history of engineering is Isambard Kingdom Brunel. But far fewer people know of his father, Sir Marc Isambard Brunel, also an engineer, who worked on the Thames Tunnel, the first underwater tunnel, described as the eighth wonder of the world when it opened in 1843.
A tunnel between Rotherhithe and Limehouse had initially been proposed in 1802 by engineer Robert Vazie to provide vital trade and passenger links across the river, since the sheer number of high-masted sailing ships meant a bridge was impractical. Another famous engineer, Richard Trevithick, took over the construction of the tunnel in 1807, using Cornish miners and their methods. But he soon hit problems, encountering quicksand that failed to support the structure. The miners, who were used to hard rock, could not adapt their methods for the soft soil and quicksand. The tunnel was abandoned in 1809.
But the company that had been formed to build the tunnel refused to give up, commissioning Sir Marc Isambard Brunel to try again in 1825. To avoid the problems Trevithick had encountered, Brunel used a tunnelling shield so that only a small proportion of dangerously soft soil would be exposed at a time. There were hiccups, however, including bouts of flooding.
Brunel enlisted the help of his son to build the tunnel, which, when completed, was large enough to be used by a horse and carriage, although it was only open to pedestrians as there was not enough money left to build entry ramps for vehicles. Trains ran through the tunnel from 1869 and it is now part of the London Overground network.
Although it is a marvel of engineering copied around the world, the London Underground's tunnels barely scratch the surface of the Earth. The Northern line, the network's deepest, goes to a depth of 68.8m (221ft) below Hampstead. By contrast, the deepest man-made mine is the TauTona gold mine in South Africa, which is 3.9km (2.4 miles) deep. But that is not the furthest that humans have bored into the Earth: the Kola Superdeep Borehole in Russia extends 12.3km into the Earth's crust.
Keeping the London Underground system operating, and modernising its Victorian infrastructure, is a never-ending task. From the continual upgrading of escalators (the first standard escalators were installed at Earl's Court station in 1911) and replacing the underground steam trains (the last ran in 1971) to rebuilding and repairing the tracks, the current operator, Transport for London, has a major job to carry out. This is, after all, a transport system that serves 1.1 billion passengers a year. That it can continue to do so, in a city the size of London, is a tribute to the Victorian engineers who began what would then have been an unimaginable network.
James Williams is a lecturer in science education at the University of Sussex's School of Education and Social Work. The 150th anniversary of the London Underground takes place this year. For more information on events and activities, see bit.ly135F1we
London Underground facts:
20.5mph - Average train speed (33kmh)
402km - Length of network (249 miles)
45% - Proportion of the network that is in tunnels
426 - Total number of escalators
55.2m - Deepest lift shaft, Hampstead (181ft)
4,134 - Carriages in London Underground's fleet
270 - Total number of stations served
19,000 - Approximate total number of staff
1890 - The date the first deep-level electric railway line opened and the name 'the Tube' was coined
1908 - The date the Tube's logo, the 'roundel' (a red circle crossed with a blue bar) first appeared