 If this is really the case it makes the cave just about 1,000 years old.Cross marks on the walls and a somewhat large niche, dug into the wall in one place and believed to be the something of an altar, are among the things that support this theory.If the Irish monks were not responsible for building Hellnahellir it must instead have been built by the farmers living in the area sometime between the years 900 and 1332 because in documents from the latter year the name of the farm, Hellar (Caves), turns up.