Hellnahellir

Hellnahellir (Icelandic pronunciation: [ˈhɛtlnaˌhɛtlɪr̥]) is the largest man-made or artificial cave in Iceland. It is located in Landsveit, South Iceland. The cave is almost 50 metres (160 ft) long and the walls are covered with engravings. Its age is unknown but it is most probably from 12th or 13th century. Hellar in Landsveit is a farm in the southern part of Iceland, approximately one and a half hours drive from Reykjavík. To those who in one way or another root from there, this place is one of great beauty, intriguing family history, happy memories and the marvellous adventures of childhood. To others Hellar is mostly known as the place where you find Hellnahellir, Iceland's biggest manmade cave.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).