Unfortunately Williamson didn’t leave a map of his tunnels. Or rather, none has ever been found. Given that he maintained a veil of secrecy around his underground kingdom, only allowing a very few to visit, it may be that he never had any plans on paper.
Therefore one has to bring together different plans which people have drawn over time since Williamson died. For example in the early 1900s the Territorial Army owned one of the large properties on Mason Street, adjacent to where Williamson lived. They used the underground tunnels for various purposes (when the Royal Engineers were based on the Street they used the Great Tunnel for underground bridge building practice!) and, helpfully for us, did draw a plan:
The whole area is bisected by the railway cutting and the long dark tunnel running left to right across much of the image is the ‘Triple Decker’ tunnel. They also drew a cross section view, showing the different layers. Note the famous ‘double tunnel’ on the right, which is accessible today.
Go back further in time and you find the hand drawn map of James Stonehouse, from 1845 – just a few years after Williamson had died and the tunneling stopped. Below, he is only drawing the main features and the map is ‘compressed’ from east to west: it should be stretched much wider to give proper proportion. But the value of what he records is immense: to confirm that such and such features existed at that date is priceless.
The railway was still in its own tunnel; it wouldn’t be opened into a cutting for another 40 years. And there’s a hint of the line of arches crossing the site from left to right. Also the huge tunnel at bottom left, which he describes in some detail in his text and part of which we rediscovered and gained access to many years later.
There are a few other drawings, maps and sources offering additional snippets. We’ve had several radar scans and core drills carried out over time and they have hinted at various features. The result is the map below, updated in early 2016. Some of the features may have been lost long ago – we’ll not know until everywhere is dug out and investigated. But it does give an idea of where everything is.