Inventory and analysis of underground oil mills in the territory of Lecce (Apulia, southern Italy)

The production of olive oil is known as one of the main agricultural activities in many Italian regions. This fame comes from a long history, and a complex chain of working phases as well, which at some locations occurred mostly underground. In Apulia, southern Italy, and particularly in its southeasternmost sector, Salento, oil mills were typically realized as subterranean spaces, due to a number of reasons, the main ones being lower expenses (when compared to the cost of building up an over-surface structure), and the easy workability of the outcropping rocks. These latters consist of soft rocks, represented by Miocene to Plio-Pleistocene calcarenites. In the territory of Lecce, the most important town in Salento, the historical documentation reports about 40 oil mills (locally called trappeti) during the second half of the 18th century. These were underground structures related to rural houses (locally called masserie), north of the town. From the structural standpoint, they belong to the typology of “cavities dug in the subsoil”, being underground structures in the strict sense: that is, rooms obtained by removing and digging rocks under the surface level. In the classification of artificial cavities by the Commission of the International Union of Speleology, underground oil mills belong to Type B: Hypogean civilian dwellings, and specifically they are classified as B.3 – Underground plants or factories. They were actually real working places, where workers often used also to sleep, especially during the hot season. The underground spaces could be very large, with a high number of rooms dedicated to collection and work of the olives, but also as storing places, stables for the animals, and sleeping rooms for the workers. In a few cases, water wells or sites to collect and preserve the ice were excavated in the same underground structure, too. Working underground had, on the other hand, negative outcomes for the overall quality of the oil, due to lack of air and light, heating due to presence of workers and animals, and the process of fermentation of the olives stored in the rooms. All of this resulted in low quality of the oil, so that it was typically used for industrial purposes (i.e., lighting) rather than for food. Starting from the beginning of the 19th century, a transformation was registered in the rural setting, since olive farming was not convenient anymore, and the passage to intensive vineyards occurred. This had as a consequence the progressive abandonment of underground oil mills, for many and many of which a loss of memory had to be recorded. In this contribution we present the results of recent geological and topographic surveys that allowed us to find 17 underground oil mills, not included in the Cadastre of Artificial Cavities of Apulia Region. Six out of them have been restored by the owners, and are presently used for different types of tourist activities; the remaining 11 are generally in bad conditions, abandoned, and often have been used as illegal waste disposal. In addition to illustrating their main characters, some considerations about the stability of the underground structures will also be presented, by describing the main situations where instability features have been observed and mapped.