[Sethomas]

Well, the most obvious explanation is that it was a loan concept from Paganism. The ancients saw volcanoes and such as gateways to the underworld, and "the underworld" became associated with Hades which in turn became Hell. People bitch all the time about how religious holidays were just co-opted from the Pagans, but that's really not a fault of Christianity so much as just a powerful tool of conversion.