Soos' Stan Fiction - Axolotl and Mexico
Soos explains what the creature living in the tank at the Mystery Shack came from, and how it got there.

Cast
Jason RitterDipper Pines (voice)
Kristen SchaalMabel Pines (voice)
Alex HirschGrunkle Stan / Soos Ramirez / Bill Cipher / Old Man McGucket (voice)
Linda CardelliniWendy Corduroy (voice)Gravity Falls on tvt

