Here There Be Monsters

Here There Be Monsters is a side quest in Fallout 4.

Detailed walkthrough
Here There be Monsters is a side quest triggered by speaking with Donny Kowalski, who's located on the docks behind The Shamrock Taphouse. He'll excitedly tell you he's been seeing an eye stalk peeking out of the water off the docks, being convinced it belongs to a sea monster, regardless of how the player may respond. Through this dialogue, the quest will begin and the Yangtze submarine will be placed on the map, located directly in front of the pier Donny is standing on. It'll take a short swim to reach and is easily visible as a periscope covered in seaweed juts out of the water and often moves up and down. This quest can alternatively be started by simply swimming to the Yangtze and does not require the dialogue with Donny to begin.

From here, the player will need to speak with Captain Zao, located in the first main room of the submarine. He questions why the Sole Survivor is there, remarking that the war is long over and tells them that he has no treasure on board to plunder. He will go on to say how his submarine ended up as it is and express surprise that he's been there for over 200 years already. He'll then tell the player his ship is not seaworthy any longer and ask if they'll help him fix it. Once the player agrees to help, he'll say that there is damage done to a dampening coil and that he needs a new one, of which his war time intelligence told him could be found in Saugus Ironworks. At this point, he can be pushed into three different rewards with speech checks of increasing difficulty. He'll start by offering Pre-War money, stating it must still have some value. He will offer caps if pushed further, and if the final speech check is passed, he will reluctantly offer his sword as a final reward.

To continue the quest, the player will need to travel to Saugus Ironworks, which is located directly south of The Slog and run by the Forged, who populate and defend the inside and outside of the building. Several members of the Forged are very likely to be wielding flamers and will use Molotov cocktails frequently. If the player has been here before, it's entirely possible for it to already be cleared out. The dampening coil is located within the Saugus blast furnace inside of the building. To reach this room, one must traverse the ground floor to the upper most floor, cross the middle of the room to the other half of the building, and then climb back down again. The blast furnace is found near the front of the second half of the building right in front of the door leading to the roof of the complex.

The blast furnace is inhabited by Slag, the leader of the Forged, and entering the room will start an unmarked encounter with him. It may be possible to retrieve the dampening coil without killing him and the others inside, but they will attack the player at the end of the immediately prompted dialogue regardless. Once Slag is defeated and the others in the room dealt with, the dampening coil can be found in the back right part of the room. Take it and leave the location, traveling back to the Yangtze.

Upon your return, enter the submarine and speak with Zao again. He'll be ecstatic at your retrieval of the item and go on to say he now needs nuclear fuel. He'll say that during the Great War, one of the missiles on his submarine did not fire and that this is what he will need you to find in the depths of his ship. He warns you that his crew is still alive in the deeper parts of the vessel and that they were not as fortunate as he was, having all been turned into feral ghouls.

Walk through the door leading to the bulkhead. Upon entering, several feral ghouls will be found roaming about. With the submarine rooms being so small, they'll need to be confronted as one travels further into the vessel. While there are many rooms, there's only ever one way to progress. Through this portion of the submarine, radiation levels can increase greatly at a consistent rate of 1 to 3 rads per second in the first room of the bulkhead and upwards of over 20 rads per second in the middle of the room where the warhead is found. It's highly recommended that one brings RadAway, Rad-X, or a hazmat suit, if not all three. As the submarine is only accessible through swimming, even with quick traveling, power armor is not a viable option, unless you have a human companion, wearing a Power Armor - the companion will appear in the submarine in his armor. Eventually, the player will reach a final room where the First Mate, who has been transformed into a glowing one, is found. This is the room the warhead is found which can be taken once the First Mate is defeated. From here, the player will need to talk to Zao once again.

Zao will tell the player to insert the dampening coil into the submarine followed by the warhead. If this is done in the reverse order, the submarine will explode, ending the quest and Sole Survivor's life. Once this is done correctly, speak to Zao one final time. He will reward the player with three Homing Beacons, each of which allows for one tactical nuclear strike, ending the quest. If any or all of the speech checks were passed at the beginning of the quest, he will also give the player 4 Pre-War money for the first passed speech check, 200 Caps for the second passed speech check, and his unique sword, Zao's sword, as the result of passing all three.

Behind the scenes
The name of the quest is a reference to cartographers often drawing sea monsters on unexplored areas of a map. In many cases, a phrase similar to "here there be monsters," such as the Roman "here are lions" (hic sunt leones), was written on the map as well.

It's also a reference to pirates of the Caribbean curse of the Black Pearl where captain  Barbosa  reveals his crews curse to  Elizabeth Swan.

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