How do I enter the Stone Skull?

After getting the Booster (and possibly the last Gear Upgrade) if you fly hugging the top of the map of Windy Plains there's a small island with a Stone skull on it.

The map shows it's a cave like the ones found in the temple and mines but when I press up on my controller it wont go in.

So how do I enter this Stone Skull?

To open the Cave of Trials, find all 42 artifacts and talk to Davy in El Machino to receive the Sigillum Caelum et Infernum gear mod.

After installing the mod, fly to the floating island in Windy Plains. Enter the skull to begin the trial.

After completing the Cave of Trials, you will unlock:

The achievement To Hell and Back and the Proof of Completion artifact.

Sources:

• Steam Community: 100% Map by Demajen
• Steam Community: SteamWorld Dig 2 Steam Achievements by alicewalice
• Reddit: Steamworld Dig 2 Endgame
• Miketendo: A (Fairly) Spoiler Free Insight into what awaits those who 100% Complete SteamWorld Dig 2!