When the London Underground was first built in 1863, the entire system used steam engines. This posed a problem with fumes building up and being stored which could potentially poison the passengers through the system. The solution was to fit the trains with condensers and then build vent areas along the line for the trains to stop and release the gasses, then continue their journey.

Perhaps the most famous vent known is the one situated at 23 & 24 Leinster Gardens in London. There are many more scattered around London which I'm in the process of locating which are still around. Originally there was a house at the location in Leinster Gardens, but the Paddington to Bayswater Line necessitated it needed to be removed and then use the 'cut and cover' method of creating the tunnels to so that the line could pass through.

It was decided that instead of having an opening on the front of the house, a facade would be used instead. The facade is about 1.8m thick and is constructed of concrete. There are eighteen blackened 'windows' (they're now painted grey) and no letter boxes present. You'll also note that the trees in the front of the house have been allowed to grow considerably more than the other trees located at Leinster Gardens. There are hundreds of people who pass this monument daily and don't even know it exists. While I was there for the second time I asked a number of people if they knew about the facade and most didn't (little silly considering the huge open space at the back of the facade!).

The address is notorious within London for pranksters and also scams. Its well known now with local business so that they're aware of when someone is joking with orders and requests. The vents have since become famous in their own right with people wanting to see the historical roots of the Underground.

If you walk around to the rear of the house, you can see the live line as shown below, with a tube train passing through and also with no train present.