Just moments ago I stumbled upon the dwarven ruin of Kagrenzel. I thought it was tiny cause it was a one room until i explored the center and lo and behold, the floor gave way and I just fell miles below the surface of Skyrim. It still amazes me the stuff you can find after 100s of hours. And for the people who fast travel everywhere, shame on you haha. This place is high up the mountain right on the border of Morrowind. It took a clever timing of jumping to scale up there. the last 45 mins was well worth it.
Just thought i'd share. I would love to hear of anything you fine folks have stumbled across.