|The following is based on Van Buren and has not been confirmed by canon sources.|
This cavern serves two purposes. First, it is the final stage of the natural purification process of river water. The water cascades down several waterfalls until it comes to rest in a reservoir. From there it is pumped up to the lake above. Second, a part of the river water is diverted to the geothermal power plant. Here the water is converted to steam to power a trio of turbines.
Upon investigating this area the player character will discover three faults. First, the pipe that carries river water to the heat exchanger in the power plant has broken. The water now flows freely into the thermal vent and is converted to steam. Second, the floor of the reservoir appears to have cracked. A whirlpool has formed and is slowly draining away the water. It will be necessary to repair both faults to bring the power plant online. Finally, the reservoir has been infested with hag-snakes like the lake above. These creatures wander freely down here and attack anything on sight.
Note: The replacement for the damaged pipe can be made at the Blackfoot town foundry.