Drive or somehow get to the lighthouse.

Then go outside to the back of the lighthouse and jump into the middle of a slit you'll find in the rocks.

After that you will see a small hole in between some of the rocks.

Go down and you will see not rock but grass (in some places, you will see some rock above you).

You aren't completely invincible, but you can get out at the bottom.

If you want to go back, simply fast travel from the bottom at bunker #2.