The doods broke into the cavern from above, and immediately they noticed something dreadful: endless giant spider webs, all over the space. At first there was some excitement in the ranks, as among the giant spider webs were also diamonds, rubies and natural silver, but they weren't sure what to do. As they returned to the surface to plan their next move, a swarm of naked mole dogs jumped into the tunnel behind them and started infesting the lower levels of the new fortress with amazing swiftness.
It was decided, rather wisely, to wall up the cavern until a military could be built to assist any mining efforts in the tunnel. On the way down, the naked mole dogs could be seen flitting in and out of the tunnels, like the Aliens in that movie, "Mary Poppins". The dwarves went insane with terror. Some locked themselves in their bedrooms, others huddles in the well room for safety, while the naked mole dogs raided the food storage facilities and glutted on plump helmet biscuits.
Using every last ounce of fortitude, the mechanic built some traps in an upstairs corridor, and the dwarves waited in subzero temperatures outside, clutching steamy mugs of heated dwarven ale, waiting for the extermination to begin.
*snap*
*snap*
*snap*
They returned downstairs to clean up the little bits of naked mole dog, walled up the caverns, and went back to their dwarvenly duties.
Meanwhile, the spores of mysterious underground mushrooms had started to sprout all over the fortress, colonizing the structure. The dwarves examined them, found some to be edible, and some to be useful for construction. They had cheated death in the horrors of the caverns, and had come out of it with some interesting discoveries, waiting for their next move.