Hmmm, no I can't say that I have heard of any Mer liches, and while I haven't heard specifically of Deepholme, I have heard of the Atmoran's use of hidden prisons. Fascinating. The night of tears does come to mind though. It tells the accounts of the snow elves invasion of the Nedic capital of Saarthal. Nobody is entirely sure why they invaded. The Mer had more or less managed to live in peace with early man and Skyrim is the only place that outright war ensued. Well admittedly it's just a theory, but if the Atmorans had captured an influential Snow Elf and held them prisoner, the elves certainly would mount efforts to get them back. If they held this individual in Deepholme adn the Elves attacked Saarthal though. That would indicate that Deepholme was quite protected and a well kept secret if that were the case. ~ Auryen Morellus
The quest to unlock Deepholme will trigger when you go to Deepholme after having completed the heist quest 'Shadows of One's Past' and set up the Explorer's Society. After 'The Vaults of Deepholme' quest has finished, Deepholme itself will become available as an off-site storage for powerful artifacts and magic items, while replicas can stay on display in the museum without being at risk of another heist.
The items can be displayed by placing them in the appropriate sorting chests. Once displayed they can either be taken from their display spots or directly back from the chests.
On the far side of the large hall is the Daedric room. On a platform above a lava pit secure behind multiple gates and the draw bridge.
The upper three rooms on left side of the hall feature the Weapon Displays. all three rooms have a weapon chest, the chests are linked together and display the weapons in whichever of the three rooms the weapon goes to.
The upper three rooms on the Right side feature Armor Displays, Again the chests are linked together and display the items across the three rooms.
In the XY room is a Jewelry Display case, a small strongbox serves as the auto-sorter for the rings and necklaces.
The bottom third room on the left (from the entrance) features a display for the three Elder Scrolls.
The bottom second room on the right has displays for the Black Books Room.
The bottom third room on the right features a large display for dragonpriest masks (could be more then just those) Deepholme - Masks Room
In order to enter after the quest, the Totemic Key is required to lower the gate.
Although Deepholme is a perfect protection against further raids of thieves, neglecting to transport all items to Deepholme will have no in-game effect.