Unfathomable Depths

Overview

 * Quest giver: From-Deepest-Fathoms - Riften Docks
 * Reward: Ancient Knowledge (Permanent Bonus - 25% bonus to dwarven armor when wearing the Heavy Armor, Smithing skill increases 15% faster :patched: 25% bonus to all heavy and light armors and smithing skill increases 15% faster).
 * Reputation gain: N/A

Objectives

 * Return the Lexicon to Avanchnzel

Notable Loot

 * Many Dwemer parts that can be smelted into dwarven metal ingots.
 * A Restoration Skill Book called the Racial Phylogeny.
 * Gems and Soul Gems looted from enemies.
 * Several chests, locken or unlocked with various loot.

Riften Docks
The Dragonborn will find From-Deepest-Fathoms wandering around Riften's docks. Upon first meeting her, she seems despondent and paranoid, immediately wanting to give the Lexicon over to the Dragonborn, pleading for it to be returned to the Dwemer ruins of Avanchnzel.

Avanchnzel
Upon arriving at Avanchnzel, four transparent orange figures will be standing at the bottom of the first large open cave room. These are the 'ghosts' of four adventurers (or "thieves", in their own words, and none of them can pick a lock), who went to Avanchnzel to recover/steal the Lexicon.

They will reappear throughout the quest and the Dragonborn can overhear that the two human characters had little knowledge about the place or item of interest, but From-Deepest-Fathoms and Watches-The-Roots were very familiar with both and persuaded them to stay and complete the job against their better judgement. The two human mercenaries stay, only because the job paid well, but it becomes clear that the Argonians sought out the Lexicon for the knowledge it housed.

Follow the marker along the corridors, the ghosts will reappear along the way. Fight the Dwarven Spider Workers, Dwarven Spider Guardians and Dwarven Sphere on the way.

Avanchnzel Animoncultory
Fight the Dwarven enemies and continue forward up the stairs and corridorr, passing a tent with four Bed Rolls used by the adventurers when alive. Beware of the trap when opening the door on the left in the next room. Continue through rooms and doors, eventually coming to a large room with two Dwarven Centurions on two large stone tables. In the next room, one of the adventurers, Drennen is lying dead on the floor. He left the group for unknown reasons.

Avanchnzel Boilery
Upon entering the boilery, the three remaning ghosts talk about the lexicon and what to do with it. Moving forward through rooms and corridors, the ghosts reappear before a door and they are in a hurry to reach the Receptacle. In the next room, the Dragonborn come up on the second dead explorer Watches-The-Roots. Finally through the last door, the Dragonborn enters a large room with the last of the explorers lying dead on the floor. A Dwarven Centurion Master is guarding the Receptacle in which to place the Lexicon. (For a lower leveled player it will not be a Master.)

Upon activating the receptacle, the Lexicon will be removed from the player's inventory and open. The Dragonborn will be rewarded with Ancient Knowledge, a permanent bonus. The Quest will now be completed.

Leave by the door behind the Receptacle and go through the barred doors and out to the Avanchnzel Balcony for a faster exit and a locked chest with some loot.

This dungeon is not accessible before the quest has been activated (Dungeon Lock Outs).

Enemies

 * Dwarven Spider Worker
 * Dwarven Spider Guardian
 * Dwarven Sphere Guardian
 * Dwarven Centurion (Master)

Bugs

 * Not necessarily a bug, but occasionally, when swimming near the docks in Riften, you will be addressed by From-Deepest-Fathoms, who irritatedly asks why you are bothering her. However, in the subtitles, her name only shows up as Fathom.
 * The Ancient Knowledge perk is currently bugged; The armor bonus does not only apply to Dwarven Armor, but enhances armor rating from ALL heavy AND light armor types, rather than to only dwarven armor; This commonly leads to the assumption that the effect doesn't work at all, since the way one expects to see the change of the armor bonus is by comparing armor rating before and after equipping different armors – this assumption is incorrect because the armor bonus is still applied, even if it isn't applied correctly. In addition, instead of increasing the rate at which the Smithing skill increases, it increases the Smithing skill by 15 points.(PATCHED)
 * This information needs some attention. On my version (first patch, PC) effect is working as intended, at least dwarven part. Increases armor values of all types of dwarven armor by roughly 25%, both equipped and not. Doesn't increase armor values of any other types of armor. Smithing part needs confirmation.
 * Smithing value does not increase by 15 points as of patch 1.4 on PC. Current smithing value did not increase above 64 after completing quest.
 * Does not increase armor values on other types of armor (PS3)
 * This is not a bug but rather a fix that was introduced later, the boost to smithing experience will be missed but the bonus to tempering armor and weapons is welcomed with open arms.
 * Smithing value does not increase, but rather the ability to smith 15% better weapons and armor (e.g. armor that can be smithed with 100 rating, can now be smithed at 115)
 * There is a rare chance that in the Avanchnzel Boilery's first ghost encounter, From-Deepest-Fathoms will not say the last line in the conversation and the ghosts do not vanish. When the next few ghosts appear, they do not say or do anything and stay put. This does not affect the quest, but it's also interesting to note that the Dwarven Centurion ghosts show up as enemies on the compass and the real Centurion cannot walk through them.
 * Saving before entering the Boilery is advised as it is prone to glitch and lock up while in the loading screen.