Underbellies have charisma. The recent discovery, beneath the Roman Emperor Hadrian’s Villa at Tivoli, of a subterranean jigsaw of tunnels and roads, has scholars and tweeters alike aflutter. Some describe this as no less than a chthonic city; others have suggested the network could stretch as far as the Eternal City itself, just over 18 miles to the west.

The presence of paved, underground streets wide enough to accommodate two-way traffic of ox-drawn carts and passages so slim that only the merest slip of a slave girl might squeeze through posits the possibility of a pallid community of slaves in this sun-drenched region of Latium. Subhumans condemned to a troglodyte existence, shifting supplies so their wine-swilling masters above could be served, invisibly.

Another compelling narrative is the fact that this “discovery” – reported last week – was made by amateurs, Italian caving enthusiasts who (one imagines) abandon wives, girlfriends and lovers after office hours to crawl through the earth in search of the past....