Abandoned mines

The 'Abandoned Mines'' are located under the Gravity Falls forest. There was an entrance under an abandoned church. The mines hold Dinosaurs from the Jurassic and Cretaceous encased in tree sap.

History
Sometime during the cretaceous period, a number of dinosaurs were somehow encased in a large amount of tree sap and preserved through the millenia. In the summer of Dipper and Mabel Pines' visit to Gravity Falls, Oregon, the summer heat started melting the tree sap, and a pterodactyl was set free. Using an exit at the abandoned church above, it started wreaking havoc around town.

Local miners thought they were about to strike a mineral deposit, but came across a massive network of underground caverns instead. While exploring the caverns, several miners went missing and died. Surviving miners reported hearing loud, high-pitched screeches, as though from a giant prehistoric creature somehow left over from the late Jurassic period. Jack Solomon, owner of the mining company, partially blamed the canary who he believed should have dropped dead at any sign of trouble. The canary was later fired.

When the pterodactyl kidnapped Waddles in front of the Mystery Shack, Dipper, Mabel, Grunkle Stan, and Soos set out to rescue him. They were later joined by Old Man McGucket, who was hiding away inside the church. Inside the caverns, the group noticed several remains of miners while exploring the area. Upon discovering the frozen dinosaurs, Stan considered making the place into a theme park. They also noticed other dinosaurs were becoming loose, and a baby pterodactyl was also born soon after they found Waddles, which ate Old Man McGucket.

The mines received some superficial damage in the course of the group escaping from the pterodactyl, and the baby pterodactyl was later eaten from the inside out by Old Man McGucket.