With a less-than ostentatious surface entrance at the bottom of the Whitehall's Clive Steps, the Churchill War Rooms is easy enough to miss.
But much like the Tardis, this series of bunkers that protected the British government and its operations during the second world war, is cavernous on the inside.
If you've visited, here are a few things to look out for next time. If you haven't been, keep your peepers peeled for these things when you go.
The cabinet room
This is where the wartime cabinet was held. Look at the clocks — they're all set to 4.58pm. That's because the first ever cabinet meeting held in this room began at 4.58pm on 15 October 1940.
Eagle eyes are needed to spot the scratch marks on Churchill's chair at the head of the table. This was caused by the tense clutching at the arms and the wear caused by his signet ring.
The fire bucket behind Churchill's chair would have been used as an ashtray. There are rumours (though never confirmed) that the Marines that guarded the room used to sell Churchill's cigar stubs on as souvenirs.
While being underground obviously helped protect Churchill and co from bombing raids, the bunker was never bomb-proof, and it's often thought a direct hit would have caved the place in. After all the bunker is only 12 feet below the surface — by contrast Hitler's bunker was 180 feet underground.
The manual calendar in this room shows the date as 16 August 1945. This is the day after the surrender of Japan, and therefore marks the last day this room was occupied as a strategic site.
There are plenty of phones in this room but the black ones with green handles are scrambler phones. Anyone trying to listen into a conversation on a scrambler will only hear white noise. It was the height of technology at the time, though the scrambler device did take 20 minutes to warm up until it was ready to use.
Look closely at the maps on the walls and hundreds of holes are visible: pins were used to mark the progress of fleets across the ocean. Some sections of map were so badly damaged by pinpricks that they had to be covered over with new material — these patch jobs may also be found on the maps.
Churchill may have only slept here three nights, but he did use this room for many afternoon siestas and was famous for holding meetings here in various states of undress.
Curtains on the wall may be drawn across the maps, in case someone visited Churchill who was not authorised to see the maps. The circles on the maps of Great Britain indicate the most vulnerable locations for an invasion.
It may seem odd to notice that the room is fully carpeted, as that's normal by today's standards. But during the war carpeting was a rare commodity and the amount of carpet in an office denoted the seniority of the person inhabiting it. Naturally, Churchill had his bedroom carpeted wall to wall.
Many of the corridors contain alcoves, but these aren't structural. They are in place in case Germany ever launched a raid on the bunker: defending soldiers could seek cover in these alcoves to fight back any invaders.
A small room with an engaged sign contained a secure transatlantic line for the American and British heads of state to communicate. Handling the phone calls would have required an early computer and there wasn't room for this equipment in the bunker. So the line would travel to the basement of Selfridges to be processed and sent on.
Originally access to this bunker was possible directly the building above. Unfortunately, that route no longer exists and a nondescript door marks the point instead. Check out the historic photo above for what it looked like back then.
Back in 2015 we were fortunate enough to go into the tunnels under the Churchill War Rooms, here's what we discovered.
The Churchill War Rooms is open to the public every day. Tickets are £17.25 for adults, concessions available.
Note: the original published article incorrectly stated that access was previously possible via 10 Downing Street and the date of the first cabinet meeting in the Cabinet room.