How can I find this "undiscovered location" in Skellige (north of Rogne)?

[JMO]

I'm am post storyline, just trying to see if I can find every single "undiscovered location" but there is one that is puzzling me.

It is here[imgur.com], the big question mark in the middle, which I have circled in red. How do I find this thing?

On the surface the point is at the very top of a mountain. And when I am there, the little down-arrow indicator appears telling me that is is beneath me. So that's not the way to go.

I've been through the "Ancient Crypt", circled in yellow at the top of the map. Did that whole dungeon and part of the main story.

I been to the monster den circled in green to the west of my characters position. And as I recall it connects to the ancient crypt. Did I miss something in there?

I've been to the bastion circled in blue on the right (for a treasure hunt),

And I've even been the place of power circled in black

But nowhere can I find a way to this question mark. Where should I look?

[Valden21]

It might be the caves where the Ancient Crypt is. Those caves are pretty extensive, going right through to Rogne. There's also a bunch of treasure chests down there; you might have missed one.

[JMO]

Found it thanks! Yes it was in the Ancient Crypts. I thought I had really cleared out that thing this time but apparently not. What I was looking for was in the room with the spikes.

One odd thing: There was another room with two of those ghosts guarding a treasure. Only now (perhaps because the main quest is over), they didn't seem to care about me at all. They just stood around while I looted the treasure.

[Ludus Aurea]

Through the circled cave.