20 Leagues Under the Sea

20 Leagues Under the Sea is a cut quest . Very little remains of the quest and what exists has been reconstructed from game files that were accidentally left in Fallout 76. 
==Overview==
20 Leagues Under the Sea would take place after completing Here There Be Monsters (identified as MS02) and repairing the Yangtze submarine. The player would join forces with Captain Zao and sail the submarine over to Vault 120, where the vessel would end up smashing into the submarine bay of the Vault. The precise circumstances of the crash remain unclear, but given the overt reference to Jules Verne and the presence of a sentient squid at the undersea Vault, it was likely that the octopod would play a major role, especially given that the squid would have been the overseer of Vault 120. The rest of the quest can be roughly plotted based on the Vault 120 cells found in Fallout 76 and the markers placed inside them, combined with leftover scripts in Fallout 4.
===Arrival===
The player would exit the stranded Yangtze by way of its forward torpedo tube, straight into the submarine bay - now with a submarine far exceeding operational parameters embedded in it (YangtzeEmergency120ToExit003 and Vault120PlayerStartMarker001), together with Zao. After navigating through the adjacent technical room, the player would reach the Vault door (it is not possible to tell whether it would be open or closed, as it behaves either way depending on whether the cells are put through Fallout 4 or Fallout 76).
Once the player opened the first door behind it (likely by interacting with a terminal, which would set the stage in motion, as per MS03AccessTerminalScrip.psc, which would also advance the quest to stage 5), leading into the screening area, the quest would likely start (MS03FirstDoorRef001). Trying to enter the office overlooking the submarine bay would trigger an attack (MS03AttackDoorRef001): An Assaultron ambush (MS03AmbushRobotRef001), encouraging the player to go in the opposite direction, deeper into the Vault.
The player would pass through the residential area, into the undersea cavern beyond the residential sector, entering it near the exit to the atrium. Judging by MS03Stepper1Script.psc, entering the area would display MS03TempHydroponicsMessage (empty in the released game), complete quest stage 100 and advance it to stage 200. 
Inside the hydroponics workshop, the player would find survivors (judging by the presence of mattresses), who jacked into the service pumps to survive. A survivor would force greet the player (MS03ForcegreetCenterMarker001). It is likely the survivor would be named James, who also has a return marker near the entrance to the workshop (MS03JamesReturnMarker001).
The player would then continue deeper into the facility, interacting with an ID scanner (MS03FirstIDScannerMarker001) at the door that would trigger a sentry bot spawn in the classroom directly above the kitchen area, adjacent to the atrium. At this point, the doors that would normally be locked and prevent access to the atrium directly from the residential area would open. The airlock leading to the passageway to the research station is tied to two additional markers (Vault120_BridgeAirlockLoadA001 and Vault120_BridgeAirlockA001).
A marker exists in the research station suggesting that the player would be able to just cross over directly. However, another marker indicates that the player would interact with Zao inside the overseer's office overlooking the atrium, accessible down a staircase right next to the airlock, leaning against the wall (MS03ZaoWallLean003). The player would then descend down the staircase to the technical area, and take an elevator down to the generator room, from which they would access the undersea section (Vault120_02toUnderwater_TEMP001).
===Seawalk===
The player would cross the generator room and, if they were not already wearing power armor, they would get one for free (there is a power armor spawn placed in the alcoves), before exiting through the airlock out into the Stellwagen Gorge, emerging by the three massive undersea generators at the opposite end from the Yangtze (Vault120_UnderSea02_TEMP001). They would then have to cross the seafloor and seaweed over to the research station, passing through two structures at the bottom of the seafloor (likely containing diver workshops, identified with the MS03Playtest14 marker).
As the walkway up to the research station collapsed, the player would have to pass through a series of underwater caves, climbing to the top of the bank directly underneath the stranded Yangtze, which would be surrounded by a powerful radiation field from its leaking reactor. From there, they would enter the research station through an airlock.
===Research station===
The way the player would navigate the research station is unclear. There is a bridge start marker (MS03BridgeStart001), which may suggest a direct entrance was possible - or that it was simply a way to note where the bridge started, for referencing scripts.
The player would navigate the research station in its entirety, discovering the grim remains of experiments or research into marine life for themselves. The stepper scripts MS03Stepper2Script.psc and MS03Stepper3Script.psc would trigger and advance the quest, displaying messages from the overseer (MS03TempOverseerMessage and MS03TempOverseerMessage2) and advance the quest to stages 400 and 450 respectively. The player would be expected to collect a password, as per MS03Stepper4Script.psc, which would instantly advance the quest to stage 600. 
James would catch up with the player once they had navigated to the top of the atrium (MS03JamesCatchUpMarker001), and follow them through the corridor to the door leading to the massive chamber with the sentient squid overseer. The door leading to the chamber has another marker (MS03FinallDLockMarker001), and it appears that the script would gather all the relevant characters inside (MS03MoveOthersMarker001).
===Confrontation===
The player would then approach the giant squid overseer, located behind the giant squid window in the background, down the central catwalk and talk to them (MS03TargetTalkToOverseer001). Zao and James would be behind the player, James to the left and Zao to the right (MS03JamesAtOverseer001 and MS03ZaoAtOverseer001). MS03Stepper5Scrip.psc would fire if the player had the password in their possession, displaying MS03TempTerminalMessage. 
The confrontation would likely lead to a decision point, likely whether to release the overseer: 
# MS03Stepper6Script.psc references MS03TempReleaseMessage which would display if the player acquired the password and activated the referenced object (with no information on what the object would be), and set the quest to stage 850. 
# MS03Stepper7Script references MS03TempEscapeMessage, which would display if the player did not release the squid (quest stage was 800 instead of 850), and advance the quest to stage 900.
In summary, the decision would likely be made regarding the life of the sentient squid overseer, whether to release it into the wild, or keep it trapped in Vault 120. James and Zao would likely represent the two options, one for containment, the other for release.
Once the confrontation was over and the player chose a side, they would head through the other exit from the chamber, to the escape capsules (MS03CapsuleMarker001), which would return them to Boston (MS03ResurfaceMarker simultaneously activating Vault120EscapePodParent; this is one of two leftover markers tied to the quest in Fallout 4).
==Notes==
* The editor ID and quest name can be found as part of the file name for the quest icon, but the corresponding values do not appear in the game files as a quest.
* An unused marker named MS03ResurfaceMarker can be found in the game data.
* Untextured assets can be found in xEdit of both an underwater vault window (VltSquidWindow01) and an underwater vault door (VltLGDoorMarine01) can be found. Unused scripts refer to this vault as Vault 120.
* Nahant would have been a major location in this quest.
==Behind the scenes==
* The name of the quest is based on the novel Twenty Thousand Leagues Under the Sea by Jules Verne.
* 
==Gallery==
Fo4 MS03 resurface marker.jpg|Position of the resurface marker
20 Leagues Under the Sea.gif
==References==
Non-game
Category:Fallout 4 cut quests
pt:20 Léguas Submarinas
ru:20 лье под водой
uk:20 льє під водою