FORGET the Shard; put aside the dome of St Paul’s Cathedral. Perhaps the greatest feat of engineering in London lies underground. Joseph Bazalgette’s sewerage network, built between 1859 and 1875, runs for about 21,000km (13,000 miles) underneath the city. A mass of tunnels, archways and cavernous spaces, the sewers are largely unknown to Londoners, though they use them every day. But on September 12th the government approved a project which may match Bazalgette’s achievement. In doing so it highlighted how much a modern city still relies on Victorian ingenuity.

Sewers have long been a problem in London. Cesspits pockmarked the city in the 14th century; in 1326 Richard the Raker, a cleaner, fell into one and drowned “monstrously, in his own excrement”. As London grew, so did the problem. By the 17th century sewage overran into the basements of houses. Bazalgette designed his sewers—a combined system, mixing water from drains with foul stuff—for a population of 3.5m, around a third more than lived in London at the time.

Some 8.4m people now live in London, and rising. Habits have changed, too, putting more pressure on the system, says Rob Smith, the chief flusher for Thames Water. People bathe more frequently; they are also awake at increasingly odd hours, lengthening the peaks of sewage flow Mr Smith and his team are used to dealing with. As the city grows, the Victorian system creaks. Last year 55m tonnes of untreated sewage overflowed into the River Thames. The notoriously unreliable British weather adds to the problem: when your correspondent visited the dripping Fleet sewer last year, high-water marks could be made out on the walls at one overflow point, far above head-height—the legacy of one particularly rainy season.