Connecting Greenwich to Island Gardens on the Isle of Dogs, this slightly forlorn passage beneath the Thames was constructed at the start of the 20th Century. In fact, it was almost flooded during the Second World War following a direct hit. The emergency repairs can still be seen today, so they say. The tunnel would normally rank much higher in this list, but is currently on (beneath?) the naughty step following irksome and ongoing irregularities in its opening times.
As Dannii is to Kylie, the Woolwich tunnel is the younger, less famous and (here opinions may differ) dirtier sister to the Greenwich tunnel. It connects Woolwich and North Woolwich under the Thames. Although it does occasionally suffer closures, the Woolwich tunnel scores a place higher than its sibling thanks to its greater obscurity and because it is kept in service despite TWO nearby alternative means of crossing (the DLR and free Woolwich ferry).
Forget the Edinburgh vaults. London has its own subterranean city of passages, crypts and vaults beneath the London Bridge area. Many readers will be familiar with the cavernous Shunt venue and the tourists-only London Dungeon. Perhaps less well known is the extensive labyrinth beneath the bridge itself, which was recently turned into the underwhelming London Tombs and London Bridge Experience. We once spent the night down there with genuine dead bodies, a bottle of wine and a ouija board. Find out how we got on.
7. The tunnel under Euston Road
We include this one simply for its obscurity. To find it, you need to wait for Open Gardens Weekend (every June) and head to Park Square Gardens (just south of Regent's Park). A short passage known as Nursemaid's Tunnel will take you from this small private park underneath Euston Road, to emerge in Crescent Gardens. If you can't wait till June, we suggest you become posh and buy one of the adjoining Regency properties to become a keyholder.
Unless you're some kind of electrical engineer, you have no chance of visiting this one. However, one of its terminal buildings can still be seen in the plaza west of the Tower of London. It's usually hidden behind an ice cream van. Look for the drum-shaped building of light-brown brick. The tunnel was built under the Thames in 1869, allowing people to cross the river downstream of London Bridge. It became redundant when Tower Bridge was completed a couple of decades later. Today it is used to carry telecommunication cables and water pipes.
With 14 exits, the Marble Arch subway is undisputed king of London's many pedestrian underpasses. Never venture down there without a ball of wool or a trail of breadcrumbs. Sadly, perhaps, a new series of pedestrian crossings at street level is gradually making the underpasses redundant. Perhaps a 100 years from now, they'll have the same 'hidden' mystique as the Tower Subway. Or perhaps not.
Waterloo, like London Bridge, is undermined by a vast network of catacombs. Many are used as workshop space, eateries and graffiti hotspots, but plenty remain unused. Back in 2009, part of this complex was rediscovered and reopened by Kevin Spacey as an unusual performance and installation space for the Old Vic theatre. It's something of a masterstroke. The most banal, ill-written performance conceivable would still impress in this space.
Once a prison, then a tourist attraction, the Clerkenwell House of Detention now lies fallow, used primarily as a film set. In the recent Sherlock Holmes film, the creepy and extensive caverns stood in as the passages beneath Westminster (you know, the ones that miraculously take our hero from the Houses of Parliament to Tower Bridge in just a couple of minutes). It's rarely open to visitors today, but be sure to book your place if the chance arises. Otherwise, watch our video walk-through.
Perhaps the most famous non-road tunnel in London, the Thames Tunnel is often cited as the first burrowing under a major river in the world. Marc and Izzy Brunel spent almost 20 years building the tunnel between Rotherhithe and Wapping, and several workers died in the process. Today it carries the London Overground beneath the river. An open weekend just before the train service started attracted thousands of Londoners.
Our personal favourite. The Kingsway underpass carried trams and trolleybuses beneath that road and out onto Embankment until the last service in 1952. Although part of the southern section was converted into the Strand underpass for road vehicles, the northern section has remained as a gaping wound along Southampton Row, gated off and inaccessible to the public ever since. The one exception was the recent Chord installation, which used the tunnel as a long art gallery. More please!
This top 10 is based on nothing more than the whims and fancies of Londonist. The capital contains many more tunnels and catacombs, so please nominate your favourites in the comments below. Extra bonus brownie points to anyone who can highlight a tunnel we've never heard of.