There are two entrances to this Cave.
The first is from Ice Lake - West Side. There are a bunch of Snow Eyes, Behemoths and an Ancient Cyclops you may have to fight to get to it.
The other is in Thule Spine. Head there from Forharn the hunter's house. Beware of 2 or more Snow Dragons on the way.
Enter this Cave with the Redshift Hammer, i.e. after completing the main quest.
Will provide a map later.