An earthquake has opened up a massive cavern under a very difficult mountain range with but one known and very dangerous pass. PCs are hired to investigate the cavern, map it, and see if it provided another way through/under the mountain. The neighboring kingdom is doing the same, and its a race to see who can reconnoiter it first, set up tariff waypoints, and scout an optimal route. Of course, whatever lives there is now also free to get out and cause mischief.