I searched and this hasn't been posted before:
Go to (20,80) Searing Gorge like this.
Then blink, and you'll be in the cave. There's nothing really here, it's just empty except for the mushrooms.
To get out, face the big rock blocking the exit and blink. You should now be inside the rock. Then go to the opposite side of where you blinked the very first time, and blink again.
Happy exploring
EDIT: Pics from inside:
To get out, face this rock (by the exit)
Then blink, and you'll be inside the rock
Here's me inside the rock
Then blink again by the next set of rocks.