Vaults of ambition: shock find under London Museum enchants its builders

When a contractor working on the site of the new London Museum at Smithfield market knocked a tentative hole in a bricked-up basement wall, all he could see, peering in with a torch, was a muddy pile of rubble and some scurrying rats.

That unpromising beginning, however, would lead to an “unparalleled” discovery. Behind the wall, once the detritus had been carefully removed, the architects and builders were astonished to find an enormous and beautifully constructed network of subterranean brick vaults that no one, even a year into the ambitious multimillion-pound building project, had known were there at all.

There had been hints on old plans of some underground structures, “but because everything was blocked in and bricked up, we had no idea that they still existed”, said Paul Williams, the principal director of the lead architects, Stanton Williams.

Certainly nothing had suggested the sheer scale of the surviving Victorian vaults, a labyrinthine forest of carefully handbuilt arches and columns that stretch across 800 sq metres – an area bigger than three tennis courts – all hiding in one of the busiest parts of central London.

If breaking through the wall was “a little bit Indiana Jones”, the discovery itself was “magical”, said Williams. Even with his long experience in the industry, he is clearly enchanted by the project to convert two historic former food markets at Smithfield , near Farringdon, into a new home for the former Museum of London, following the closure in 2022 of its previous home at the Barbican.

Part of the new museum, due to open in 2026, will be situated underground, at the same level as former Roman streets, the passing Thameslink trains (which will be visible to visitors through a window) and the hidden River Fleet that flows behind thick brick walls just metres away.

“Everything we do, this whole project, is about imagination,” said Williams. “If anything triggers the imagination, it’s trains, it’s the Romans, it’s vaults you can’t find. It’s about hidden treasure, which is so right for the London Museum.”

Having had some time to research them, the museum’s senior curator of archaeology Francis Grew knows a little more about the vaults’ history and purpose. They were built along with the general market in the 1880s, partly to help hold up the busy Farringdon Road above, but also, he thinks, as storage space for the huge volumes of goods that were coming in and out of the markets on what was then the busiest stretch of railway in Britain.

“But one of the most curious things is exactly what these vaults were intended to be used for,” said Grew. “That’s where the real mystery and the interest and the excitement is – they’re still not fully disclosing their past history.” Fruit, vegetables, meat and textiles were probably kept here, he said – but the vaults may even have been used as stabling for the many packhorses that worked at the markets.

Their meticulous construction is more than just structural. Williams points to the use of rounded, lighter-coloured bricks on every column edge, which he believes would have helped porters navigate the underground maze in poor light. “This is cathedral [-level] building. It’s remarkable.”

Finding the undiscovered vaults may have been a delight, but it also caused a huge headache: they were not incorporated into the ambitious scheme that had won its architects the renovation contract, and certainly did not appear on any budget.

“We were shocked in the first instance, and very quickly then racking our brains thinking: what do we do about this challenge?” said Joe Kenway, who leads the scheme’s project management on behalf of the museum. Ultimately they felt they could not ignore this opportunity, and additional funding was sourced from the City of London Corporation to incorporate the vaults into the new museum plans.

Five years after that first breakthrough moment, the vaults’ huge expanse of brickwork has now been meticulously restored, the floors and lighting are about to go in. Eventually, museum visitors will be able to explore the space for themselves, though initial plans were not to use them to display the permanent collection but to host pop-up events such as immersive theatre or large-scale dinners, said Kenway.

“I think, for the museum, this really deserves to become one of London’s really special places,” he said. “Loads of people that we speak to on the project have family connections to people who’ve worked in the Smithfield area spanning back over quite a long history. And I think by making this space accessible to the public, we will be giving those people, and national and international visitors, the opportunity to connect with this really special part of the city.”

• This article was amended on 19 August 2024 to remove an incorrect reference in the subheading to the project costing £150m. Also, part of the museum will be situated underground at the same level as passing Thameslink trains, not Crosslink trains as stated in an earlier version.