There is a Wikipedia article for this location.
Mimer's Well
Mimer's Well is located in Mimer's Realm.
The main reason to visit is the quest for Heimdall's horn Gjall, obtained by defeating Aspenth at the bottom of the well.
In order to embark on this quest, some prerequisites are in order:
- You must be able to swim.
- You can't be carrying too much weight, or you won't be able to resurface. On the other hand, you can easily dimension travel regardless of weight.
- You can hold your breath underwater; this is related to your Constitution score. (You'll be warned if your air is running low.)