The Skull Cavern is in the northwest corner of the Desert in Stardew Valley, behind the locked entrance that opens with the Skull Key. To get there, you also need access to the Desert via the bus.

How to reach it

  • Go to the Desert.
  • Head to the upper-left / northwest area.
  • Use the Skull Key to enter the cavern.

Unlock requirement

  • The Skull Key is found on floor 120 of the Mines.
  • Once you have it, the Skull Cavern entrance in the Desert becomes accessible.

Quick note

It’s a much tougher dungeon than the normal Mines, and it has no elevator, so each run starts from the top.

TL;DR: Northwest Desert, unlocked with the Skull Key.