How many of these little-known spaces beneath London did you know about?
The salad farm beneath Clapham
Deep, deep down, beneath the pavements of Clapham, the Growing Underground company nurtures salad leaves and herbs in its vast hydroponic bays. The unorthodox farm makes use of a deep-level shelter from the second world war. We visited in June 2015.
Many disused tube stations can be found across the tube network (map here). The former Piccadilly line station of Down Street holds particular interest. It was closed in 1932, but recalled to life during the second world war as a bunker and control centre. We visited in May 2015. You might also enjoy our earlier visit to Brompton Road ghost station.
London's deepest tunnel
The Lea Tunnel was formally opened in January 2016. It's a relief sewer, taking up waste that would otherwise be piped into the River Lea. It is also London's deepest tunnel, at 75 metres. We visited in April 2015.
A paddle in the River Fleet
The most famous of London's buried rivers, the Fleet now serves as a sewer running from the slopes of Hampstead Heath down to Blackfriars. We've twice taken a paddle, in August 2010 (Farringdon) and October 2015.
The bunker beneath a bunker
You've probably heard of the Churchill War Rooms, the visitor attraction near Whitehall that recreates a wartime operations room. The public areas are just a small part of the complex. Deeper down lurks a further catacomb of disused passages and rooms, which even incorporates a buried river. We visited in February 2015. You may also enjoy the Battle of Britain bunker in Uxbridge, occasionally open to the public.
The sublime subway
Two tunnels under the river you probably don't know about
The Thames Barrier hides a subterranean secret. Twin tunnels run the width of the river, to allow engineers easy access to each pier. We visited in January 2015.
A first ride on Crossrail
London's new underground railway, recently christened the Elizabeth line, won't open in full until late 2018. For those with special access, however, it's long been possible to ride a train through the tunnels. We visited in February 2014.
Alongside the sewers and tubes, London contains many sizeable utility tunnels. National Grid recently constructed 32 km of tunnel beneath the capital. We visited in February 2016.
London's other tube system
The Post Office operated its own private tube line between 1927 and 2003. The so-called Mail Rail ran between Paddington and Whitechapel, transporting parcels across the capital. It has always been strictly off-limits to the public, but a short section beneath Mount Pleasant will open for tours from spring 2017 as part of the new Postal Museum. We visited in April 2014.
Bonus bunker: a cave system you can visit
Chislehurst Caves in south east London include 22 miles of ancient man-made tunnel, plus heaps of history and legend. We visited in August 2015.