I have the Skull Key. How do I open Skull Cavern?
Two different unlocks stand between the farm and the first cavern floor. The key opens a door in the desert; it cannot repair the bus that carries you there.
Collect the key in the mountain
Descend through the ordinary mountain Mines and open the chest on floor 120. Elevator checkpoints make this a multi-day goal: reaching 115 lets you begin near the final stretch on your next trip. Pick up the Skull Key and check Special Items & Powers in the current game menu. It is a permanent unlock rather than a tool you equip from the hotbar.
Restore service and buy the ticket
On the Community Center route, finish all four Vault payments. Their values are 2,500g, 5,000g, 10,000g and 25,000g; together they take 42,500g. A Joja member buys the Bus repair separately for 40,000g instead. After the overnight repair, use the ticket machine at the Bus Stop when service is available. Keep 500g for the fare. Clicking the bus driver for ordinary conversation does not buy the ticket.
Use the desert door
On arrival walk toward the cave mouth in the northwest part of the desert. Inside is a locked door, distinct from the outside cave opening. Interact with it while your character has the key, then take the ladder to begin level 1. If the bus runs but the door remains locked, revisit your floor-120 reward and check the permanent-items tab; raising friendship with Pam does not award a key.