Souterrains are man-made underground or partly underground passages.
There is a souterrain in the hillside between Staigue Fort and Staigue Fort House. Access to this souterrain is blocked by stones. From the entrance of this souterrain, there is an uninterrupted view of the fort in the distance. It has not been excavated. The ceiling of the entrance chamber is made a long flat rectangular stones. There are 3 further passages leading from the entrance chamber.
The term Souterrain comes from the French: sous – under, terrain – ground. There are only 1,000 or recordings of these structures in Ireland. They have been found in ring-forts but also feature in a variety of other medieval sites e.g. promontory forts and open settlements.
Similar, though not identical, underground structures have been found in Brittany, Cornwall and parts of Scotland. There is no evidence that there were links between the various groups.
Two disparate styles of building technique were employed – one where the souterrain was dug into the earth and another where the land was rocky, in which case the souterrain was built wholly or partly above ground, being dug into a rath or built as part of a cashel enclosure.
A simple souterrain, built using the style suited to rocky terrain, could consist of an upper passage leading from the entrance to a small chamber at the far end, a vertical shaft in the passage floor, known as a ‘drop-hole creep’ leading down to a second more spacious passage. A number of passages could be joined by these creeps. Passing from one passage to the next would be difficult as the headroom was lowered.
It is speculated that souterrains were used for storage or for refuge. Some souterrains have ventilation shafts. There is some evidence that wooden doors were used to close creeps. Cupboards, shelves and benches have been found in souterrains. Using the scientific method of dating based on the analysis of patterns of tree-rings: dendrochronology, it would be possible to the tell when the timber used for such furniture was felled.