Europe's subterranean spaces are among the continent's most puzzling places. Witness the mystery that surrounds Tiscali in Sardinia, where, in a great limestone cave on the eastern slopes of Monte Corrasi, there are the remains of an underground city. Even in an island as rich in archaeological heritage as Sardinia, there is little to match Tiscali, an entire community built underground. Elsewhere in Europe, there are other magnificent underground cities, silent relics of communities consigned to history.
The underground towns of Cappadocia in Turkey are creations of fabulous complexity; in their heyday, the largest, such as those at Derinkuya and Kaymakh, may have been home to more than twenty thousand people. Today, visitors to Derinkuya can explore eight subterranean levels of workshops, housing and community facilities, all serviced by ventilation shafts that penetrate to more than seventy metres below the surface. Elsewhere in this region where Europe blends gently into Asia there are many other cave cities - over a dozen in the Crimea region of Ukraine, the complex of thousands of small caves at Khndzoresk in southern Armenia, and the three storey cave city at Noush Abad in central Iran, only discovered last year.
All these subterranean cities were communities of the living. Throughout Europe, there are also spectacular underground empires of the dead. One of the largest catacombs anywhere in the world is that in Paris, for example, by the Denfert-Rocherau métro station. How many commuters notice the entrance to the subterranean world that houses the mortal remains of several million Parisians? In the Mediterranean, there are fine catacombs in Cyprus, Crete, Malta and Sicily. Each has its own distinctive architectural style. The Christian catacombs around Rome's Via Appia Antica are justifiably famous and much visited. But even in Rome, there are dozens of catacombs and necropolises beyond those much celebrated in the regular guidebooks.
Adam J Shardlow has been checking out what lies beneath the surface of The Eternal City.