They could be as small as a closet or they could continue underground for quite a distance. Some archaeologists think they were used for hiding from enemies but the most common explanation is that they were used for food storage.
The underground temperature stays fairly even and in pre-refrigeration days these souterrains would be perfect for salted meat, grain, and wine.
The walls and ceiling of the souterrain were lined with stones.
This is another photo deep inside a souterrain. I found this photo on Flickr and I don’t know the man in the picture but he doesn’t look very comfortable! Note the water standing in the floor of the souterrain.