Pandora's cube is a "rare" item that scatters in different maps in Trails Frontier. They are usually hidden very well inside walls themselves, with usually a secret entry and exit (if there are any exits). A good sign of a secret exit are small bumps on the sides of the rocks that appear in the center of the trail. The doors may be of a different shade of gray than the surrounding rocks, presumably due to a error in the code. Known locations (of the cube) include but are not limited to Fort Ruins, Rock Wall, the craneyard and the lift.
Upon finding the first cube, a pop up window featuring the shop owner will show up, and after the race a mission in saloon will be shown, where the player will receive 1000 coins and 10 gems.