I had not heard of the Caves of Nottingham before, but there is a lot of geology near/under many urban areas around the world that host their own underground city as a mirror, parallel, or predecessor to the current above ground city.

While not as extensive as the recently discovered cities in Turkey, it still shows that such places existed far and wide.

How likely are such caves to be forgotten and discovered by accident, or have cave-ins?

How likely are the things that now live in the caves to decide to explore the outside world for fun, profit, or food?

A sample of what the Medievalists.net describe about the Nottingham caves:

The Anglo-Saxon writer Asser referred to Nottingham as Tigguocobauc = “the house of caves” and some of the caves date back over a thousand years. During the Middle Ages, some of the caves served as a tannery, chapel, kilns for malt and pottery and a secret entrance into Nottingham Castle.

More caves were created in modern times. Historic England reports, “The Victorians also used the caves as stables, for cold and fireproof storage, or as tourist attractions, follies, and summerhouses. In the 20th-century there were catacombs, garages, and air-raid shelters. There is even an underground skittle alley, with a slot carved in one wall for your ball to return through.”