Finally I've found it! With the help a some local knowledge, it has to be said, but even then it's easy to walk right past the landward entrance.
So, I'm not going to tell you where it is, but if you're out and about in that area and interested you'll figure it out. Some things are worth the search.
So here's a little video that should give you a few clues as to where it is. Mind, there's no smuggler's loot in there!