🇹🇷 Turkey · Underground city
Kaymaklı Underground City
Hittite to Byzantine · Phrygian / Byzantine
Second major Cappadocian underground city, 8 levels open to tourists, rolling doors, stables, wine storage, connected to Derinkuyu via tunnel.
🇬🇧 United Kingdom · Megalith
Mine Howe
Iron Age (c.100 BCE–400 CE) · Atlantic Iron Age (Orcadian) / possible Roman contact
Iron Age underground sanctuary in Tankerness with 29-step stair descending 7.5 m to a corbelled chamber.