TL;DR: A previously unknown man-made cave from the Viking era has been excavated in Iceland. Man-made caves in the region were first found in 2018, but the latest discovery was a much larger cave connected to the wider cave system. The cave may have served as a stall for cattle and horses.

https://www.heritagedaily.com/2022/05/man-made-viking-era-cave-discovered-in-iceland/143731