Netherworld

The is a train network underneath Tokyo, which the majority of was destroyed during the Great Global Flame Disaster.

Overview
As it's forbidden to enter, one must gain approval from The Holy Sol Temple to enter, though it's not needed to gain entree as in the case of the Ash Flame Chivalric Order residing there. The temple rarely allows individuals to enter, and it teaches its members that it's an unclean land which is hell for not receiving the light of the Sun God.