The characteristics of the rocks outlining the complex stratigraphy of the Rupe on which Orvieto stands, determined the environmental conditions that led its inhabitants to dig underground until they formed the incredible maze of grottoes, wells, cisterns and tunnels that today take the name of Orvieto Underground.
Seen from afar, for example when arriving by motorway or going down the sharp turns of the main road 71 coming from Viterbo, Orvieto has a very particular appearance, immediately intriguing thanks to a remarkable sight. At the southern end of a wide valley enclosed by hills, some of them wooded, some neatly lined by vineyards, it stands out like a relief in perfect isolation, its slopes rising softly from the bottom of the valley. At the top, almost as though planted there by a mythological giant, a vast tuffaceous block with tall and smooth perpendicular walls supports the city which with its roofs, towers, domes and spires, breaks and stirs the fairly level surface of this unexpected plateau called “Rupe”.
The stratigraphy of the charming structure is somewhat complex but, starting from the bottom, it can be simplified into the following three layers:
- the hill, formed by Pliocene clays, the old seabed present here before the emergence of the Apennine range;
- the “Albornoz series”, a thin layer not visible everywhere, originating from deposits in a fluvial-lacustrine environment including layers of volcanic origin, so called because it emerges with greater emphasis near the fortification erected in the XIV century by Cardinal Albornoz;
- the actual Rupe formed in the course of about three hundred thousand years from what was the final eruption of the complex volcatno of the Volsini Mountains. It is composed of lithoidal tuff with black scoriae including real yellow-orange tuff, of lithoidal texture with numerous inclusions of pumice, and extremely friable rock of a grey colour that incorrectly takes the name of pozzolana.
This particular stratigraphy influences the circulation of underground waters and observing helps to understand why, over the millenniums, the inhabitants of the Rupe worked so peculiarly in the city’s subsurface, so much that they dug about 1200 grottoes.
Both the tuff and the so-called pozzolana, due to their natural porosity and the large presence of fractures, appear to be completely permeable, which is why the meteoric waters that fall on the Rupe don’t encounter obstacles as they go down to the impermeable layer of Pliocene clay.
This is where the sub-horizontal flow begins giving rise, all around the large tuffaceous block and right at the junction point between the clayey layer and the upper layers, to various springs. Therefore all the water available to the built-up area has always come from outside of here or rather, from outside the vertical walls of the Rupe, always considered by Orvieto’s ancient inhabitants as insurmountable protective walls. It is difficult to imagine how, beyond the difficulties of everyday provisioning, this situation could be extremely dangerous in the event of a siege: it would have been impossible to leave the city and reach the springs now fallen into the hands of the enemies. The unbreakable need for a water supply was therefore the incentive that probably set in motion the realisation of Orvieto’s underground world. Elaborate cisterns were thus dug to hold rainwater and deep wells that, once the permeable layers were overcome, reached the phreatic surfaces.
Along with these hypogea, pertaining purely to seeking and holding water, all the other grottoes were dug too, for a vast range of needs. In this case too the special geological nature of the Rupe came to their aid: the insubstantiality of the pozzolana and the inadequate hardness of the tuff made it possible to make the best use of Orvieto’s subsurface, thus creating that surprising “double” of the city, Orvieto Underground.