A Souterrain is a type of underground construction mainly associated with the Atlantic Iron Age. Built near settlements, they were used as storage for food or for protection through hiding from raiders. After being dug out they were lined with flat stones, with staircases down into their depths. Of those excavated throughout the UK and Ireland, artefacts are rare, indicating that they were merely in use temporarily. Some are very small, while others resemble passageways; my guess is that it would depend on the size required by the settlement, and how much time they had to prepare it.
I came across a souterrain along Loch Eriboll in 2012, as I was in the area doing research for a novel I’m working on. In these photos you can see that, if you were walking out there at night or dusk, they could be very treacherous. My husband crawled down inside to take a picture back out; it was roomy enough for him to stand once inside (in this particular souterrain even the ceiling was lined with large stone slabs), though the narrow stone stairs and proportions in general indicated a much shorter population than modern humans. you can see from the photo of myself how overgrown the bracken and heather is; it was nearly completely hidden; we were looking for it based on a geological map’s markings, but if we hadn’t known it was there, it would have gone completely unnoticed.