You find yourselves being sealed into the caves by the bandits. You weigh your options: Try to dig out the barracade that the bandits have made, find a secret door, create a portal, or find a portal.
You cautiously approach the pool in the caves and discover that it is an illusion. Stepping into it one at a time, you vanish, and reappear in a deep cavern above a whirlpool into which you fall.
The whirlpool takes you down an underground river to an underground lake. The only light in the cavern appears to be coming from a proliferation of mushrooms and fungus. You leave the lake and notice many miles off a glow that appears to be some sort of fortress. From your knowledge of history and dungeoneering, you suspect that it is the moving fortress of Hraak Azul. An enormous fungus that slowly crawls through the underdark. It is inhabited by troglodyes that hunt creatures for food. You feel that reaching there is your best hope of finding a way out.
Along the way you encounter a patch of large mushrooms that turn out to be predatory myconids bent on digesting you for lunch. You manage to best them, and then move on.