I wonder what will happened if this keeps up?

Actually, I´m wondering why did Kasen get upset about the geyser being dangerous to humans? Isn´t that place far, far deep into underground, anyway?

What humans could possibly go down there (or get there alive) other than the protagonists?

the geyser connects to the surface, where the hot springs are.

She keeps bringing up the issue of criminal ghosts. I'm not sure if I misunderstood previous canon, but if doesn't that mean they were already sentenced by Shiki and in hell? I know I should blame Komachi but I don't know if criminal ghosts means, "Ghosts of bad people" or "ghosts of the damned"

I think she's more concerned about evil spirits, and that 'noxious gas being harmful to people' is just excuse.