The passage from the Jotun Lake at Base 701 leading to the Unstable Danuar Mysticarium keeps opening and closing.

After it stays open for a couple of hours, it disappears again.

To create a key to enter it, you need the Base Crystal of Bases 701 and 712.

There is a scout who went to Jotun Lake to find out more about this. If you take the Ruined Red Crystal Key to Palarus, you can see if he's learned anything new.
The research on the Crystal Keys must be complete.

I also found out a bit more about the intervals in which the Unstable Danuar Mysticarium opens.

They seem to be about 2 hours apart.

And it takes about 15 minutes until they disappear again.

I know it's not much, but please tell the Tribunus anyway.

Player: End dialog.

So he discovered more accurate information regarding the intervals and durations.

The riddle seems to slowly solve itself.

If you are so inclined, Player, you should go to the Unstable Danuar Mysticarium yourself.