The Ploutonion at Hierapolis, Turkey
The Ploutonion at Hierapolis in Turkey is a religious site dedicated to the god Pluto. The cave below the temple is home to large amounts of carbon dioxide that are powerful enough to kill most living things instantly. It was seen as a ‘gateway to the underworld’ and was often a place of animal sacrifice.
It is not technically unexplored – religious priests in Roman times would hold their breath or avoid the deep hollows where the carbon dioxide lingered as they ventured to prove their spiritual status – but remains an interesting relic and is still incredibly dangerous to approach today.