The Great Outdoors

Chicagoan Chester "Chet" Ripley (Candy), his wife, Connie (Faracy), and their two sons, Buckley "Buck" (Young) and Ben (Giatti), are on vacation at a lake resort in Pechoggin, Wisconsin during the summer. All is going as planned until Connie's sister, Kate (Bening), Kate's investment broker husband, Roman Craig (Aykroyd), and their twin daughters, Mara and Cara (Rebecca and Hilary Gordon), crash the vacation.
Ghost stories at the family BBQ include one of a man-eating grizzly bear that Chet met face-to-face when he was younger. Chet says that while he and Connie were honeymooning at the same lake, he was attacked by a giant grizzly bear. When Chet fired at it with a shotgun, the buckshot shaved the hair off the top of the bear's head and from that day on, it was known as the 'Bald-Headed Bear' of Claire County.
After Roman pulls Chet around the lake on an impromptu water ski ride with his rented speedboat Suck My Wake, tensions between families erupt. Chet is ready to pack up and go home, even as teenager Buck tries to romance local girl, Cammie (Deakins). The budding romance goes well until Chet is challenged to eat the Old 96'er (a 96-ounce steak) at a family dinner which causes Buck to break the date. Buck tries to apologize to Cammie for being late, but Cammie refuses to speak to him.
Connie and Kate bond at a local bar when the conversation drifts to Kate's challenges of being wealthy. Later, just at the peak of tension between families, Roman offers Chet a piece of his newest deal culminating in Chet writing a $25,000 check (more than he and Connie have). As the Craigs drive away, Roman abruptly heads back and reveals to everyone (even Kate) that he is broke, having lost everything in a bad investment.
Later, during a thunderstorm, the twins wander off and fall into a mine shaft. Chet and Roman eventually find them, but the claustrophobic Roman is reluctant to descend into the tiny mine shaft. After some encouragement from Chet, Roman summons up all his courage, while Chet goes for rope to pull them out. Upon realizing that the mine is stocked with old dynamite, Roman takes his daughters and climbs out of the shaft on his own.
When Chet returns with the rope, he is horrified to discover the 'Bald-Headed Bear', lurking in the mine. It chases him back to his house, smashes through the door, and rampages through the house. Wally (Prosky), the campsite owner, bursts into the house with a loaded shotgun (lamp) while Roman tries to hold off the animal with a fire poker and an oar. Chet takes the gun and shoots the bear, blowing the fur off its backside and leaving the bear's bottom exposed. Roaring in embarrassment, the bear runs out of the house.
The next morning, the two families part on good terms. Unbeknownst to Chet, Connie has invited the Craigs to stay with them until they can get back on their feet. Cammie and Buck make up and end their summer romance, as Buck and his family head back to Chicago.
In a post-credits scene, a pair of raccoons (two of a pack which had been ransacking the garbage of both Ripley and Craig families throughout the film) discuss the "Bald Headed bear" having been shot in its rear end, with one raccoon confirming that the bear was "bald on both ends now".

Question: What do the twins wander off and fall into?
Answer: a mine shaft