These underground chambers were built in the 19th century to house the horses and pit ponies that used by the railways to deliver and pick up goods for transportation. An extensive network of tunnels, from Euston mainline station all the way up and under Camden Lock market, and the Primrose Hill rail depot. You can trace the route via iron grills set into the roads, allowing light and air into the network of underground tunnels.
There is even an underground lake, or canal basin, providing water and - probably unintentionally - access to urban adventurers who like to enter the tunnels in this way and do their own, unauthorised, exploring.
The Camden Catacombs - another example of London's extensive portfolio of underground spaces that The Old London Underground Company are assisting.