The Eternal Laboratory
It used to be a separate area instance, but it is removed in version 3.0.0.
Despite the layout of The Solaris Temple Level 2 in Act 3 and Act 8 is largely the same, this subarea is absent from Act 8 version. The door to the subarea is closed and not interactable.
It contains a crafting bench recipe to unlock.
screenshot of the area before version 3.0.0
The Eternal Laboratory (end game version) was used to be accessible once you have completed the quests for Lady Dialla in Solaris Temple Level 2 in Act 3 on Merciless difficulty. You can then use a waypoint to get there. Zana, Master Cartographer was also found here after completing your first map.