And he believed some were still being maintained.
He said: “It is an astonishing time capsule down there, probably the Seventies in terms of decor.
“It feels like an underground space station, almost, of these winding tunnels that go on and on forever but are filled with dusty equipment from when it was used for Cold War communications.
“The bits we went into were certainly abandoned, though they still had lighted on when we were in them, so it was being maintained to some extent.
“There was certainly lots of water dripping off walls and rust everywhere, but the actual deeper level tunnels, I suspect, were being used, otherwise I’m not sure why they would have bricked off access to them.”
Author Mark Ovenden believes some of these structures date back to the Industrial Revolution when London became the busiest and most modern city in the world in the 18th century.
Land Registry documents were released (Image: HM LAND REGISTRY)
There was reportedly much more of a need to use the spaces below the city than there had been pretty much anywhere else.
He told the BBC in 2020: “The city was so huge that they couldn’t smash down the centre of the already built-up area, so they had to build under it.
“It just had that incredible number of people living there for such a long time during a period when technology was enabling more tunnelling to happen.
“As you place your foot on a paving stone in London, you are literally walking above hundreds of miles of tunnels and passageways and ducts and sewage pipes and chambers and secret places that most people have absolutely no idea are there.”
Project Iceberg, a collaboration between the Connected Places Catapult, the British Geological Survey and the Ordnance Survey is attempting to bring together all of London’s subterranean features.
There are many tunnels below the capital (Image: HM LAND REGISTRY)