Samuel Hansen directed me to this article: “The secret cave city under Nottingham“. The article is highlighting The Nottingham Caves Survey, who offer:

The Nottingham Caves Survey is in the process of recording all of Nottingham’s 450+ sandstone caves. The project is now underway and we are surveying caves even as you read this. Keep checking the website for newly- surveyed caves! You can read more about the caves, see photographs, watch fly-through videos and take virtual tours.

The survey is producing some interesting images, such as the following (credit: Trent & Peak Archaeology / The University of Nottingham; click to enlarge):

Laser-scanned image of King David’s Dungeon, below Nottingham Castle. King David II of Scotland was reputedly held captive here in 1346.

Laser scanned orthographic plan of the Goose Gate caves, Nottingham. These caves include a medieval malt kiln, 18th-century brewery cellars and a 19th-century butchery.

And some very compelling videos:

I’m not sure about the use of the phrase “secret cave city” and the talk of the Survey using techniques to “reveal elaborate cave systems under the town of Nottingham” (in the post, not used by the Survey itself). I am pleased that the Survey is producing detailed scans of these caves, it’s interesting business and nice to see them being made accessible through multimedia, but the caves were not previously secret. In my experience, the existence of widespread caves under Nottingham is fairly well known. The Survey knows this and is specifically scanning those caves that are physically accessible (although perhaps not all open to the public).

Nottingham is referred to by a biographer of Alfred the Great in 868 as “Tigguo Cobauc” which you see translated as “House of Caves,” “Place of Caves” and “City of Caves”. A book on the history of Nottingham I have has the following:

“Entering the town at the beginning of the seventeenth century, Bishop Corbett exclaimed: ‘Why, the people live not in howses but are earthed in holes!’ If a man was destitute, explained one writer in 1639, ‘he has only to go to Nottingham with a mattock, a shovel, a crow or an iron, a chisel or a mallet, and with such instruments he may play the mole, the coney, and work himself a hole or burrow for his family where over their heads the grass and pastures grow, beasts do feed and cows are milked.’ The caves in the area of the castle have been used as mushroom farms, cockpits, gambling dens, privies, drinking cellars and burial vaults. During World War II, some factories enlarged them as air raid shelters for their workers. Today the caves house social clubs, a rifle range, store-rooms and a cosmic-ray research centre. From time to time somebody gets even more exotic ideas for them – such as linking them together for a Tube railway or turning them into atomic shelters, to be reached quickly by people sliding down on mats as if on a helter-skelter. Builders, of course, regard them as a nuisance and a hazard, and refuse to give a firm quotation for work in the centre of Nottingham. But the tours of underground Nottingham run for the 1972 festival proved so popular that the city has woken up to its subterranean history as a tourist attraction. The thirteenth-century tannery (the only underground one in Britain) revealed in the multi-million pound Broad Marsh development, has been spared”
(Portrait of Nottingham by E. Bryson, 3rd ed. 1983, p. 26).

I reckon there’s a good alternative history story in those imagined subterranean Thunderbirds-esk helter-skelter cold war bunkers.

The quote above is all interesting but the phrase “cosmic-ray research centre” in particular catches my eye. There is a paper ‘The Absolute Intensity of Muons at 31.6 hg cm^-2 Below Sea-Level‘ by Crookes & Rastin (Nuclear Physics B 58 pp. 93-109, 1973) which describes an experiment in a cave under the Castle:

“A cave in bunter sandstone of uniform composition lies below Nottingham Castle, and this provides an experimental site at 43.0 hg cm^-2 from the top of the atmosphere. The apparatus used by Crookes and Rastin to measure the absolute intensity at sea-level in both the vertical and inclined directions was therefore used to provide similar data at this underground location” (p. 96).

The first picture above shows some of the caves under the Castle and it seems some of them provided an experimental site for detection of cosmic rays for physicists from the University of Nottingham in the 1970s.