Combination lock

Miner says that he knows about the lower door of the Monolith bunker the stalker in Captivity was looking for. The code for the second is 434018, but there are actually three code-locked doors - the third one is in between the others. A fanatic named Shadow of the Monolith holds the code for one of them.
Miner has a plan to open the door. You will have to disguise yourself as a Monolithian, enter the Labyrinth, and pay Shadow of the Monolith a Plasma Caterpillar and Rattle (as he's rumored to be an avid artifact collector) to find out the code, open the door, grab the safe and exit. Sidorovich can help you with the disguise.
Miner also warns that you'll need access to CNPP-2 before talking to Sidorovich and starting this quest.
== Walkthrough ==
=== Preparations ===
During this quest and subsequent ones, you'll have to visit certain locations as a Monolithian. This means all factions except Scientists, Traders, Avengers, and zombified stalkers will be hostile towards you. Getting through the storyline unscathed requires some careful route planning.
At minimum, ensure you have these before starting the quest:
* A Monolith exoskeleton. You must wear it during this quest and several subsequent ones.
* Plasma Caterpillar and Rattle, to give to Shadow of the Monolith.
* The best bullet protection in artifacts you can get.
* At least 50kg of spare carrying weight.
* Transition from Agroprom to Cave. (Mole's PDA)
* Transition from Cordon to Agroprom. (Tricky stash)
* Transition to CNPP-2. (either A Farewell Note or Golden chunk)
If you don't want to spend any money teleporting around, make sure you have these transitions:
* from Agroprom to Yantar. (Rat race)
* from Yantar to Radar. (found in Radar)
The following are not necessary, but will make the quests easier:
* 5-10 Smoke grenades.
* Teleport to Army Warehouses, obtained in Strelok's PDA near Wish Granter.
* Two Monolith teleportation devices, obtained for Controller's brains or Skinflint's cyclic quest.
* Teleport to the Cordon ×2.
* Transition between Sarcophagus and X-10.
* Secret tunnels between Central Pripyat, Bar, and Cordon unlocked after grabbing extra Strelok's stashes.
* Jumping artifacts, in particular Octopus and Swamp Phantom.
* A Cloaking exoskeleton and a fully charged High-capacity battery to power it.
* Extra Body armor repair kits, as the Monolith exoskeleton has extremely low durability.
You'll also have to carefully monitor your reputation before and after the quest.
* Make sure you never attack any members of the factions that are normally neutral or friendly. Doing that will make them permanently hostile after the quest. You can shoot bandits, militaries etc. without any problems.
* Don't travel through the Freedom base at Army Warehouses and don't let yourself be seen by them. They are special, and even seeing you as a Monolithian will drop your reputation with Freedom. This is not the case for other Freedomers, only for those at the base.
* Before the quest, write down your reputation with the major factions. After completing the quest, verify that it didn't drop.
=== Joining the Monolith ===
Travel to Sidorovich. He will agree to give you an amulet of a Monolithian leader - wearing it will mark you as a member of the faction. You will also need to wear a Monolith exoskeleton to complete the disguise.
However, he has one condition. You have to look for boxes with special repair kits that are kept in the Labyrinth outpost among the containers. Until you bring them to Sidorovich, you won't be able to rid yourself of the Monolith medal. This is tracked in the Repair boxes quest.
=== Entering the Labyrinth ===
You have to travel to the Labyrinth entrance somehow. 
* Exit Sidorovich's bunker.
* Evade the Rookie village by rushing or sneaking aside.
* Cross the railway in any available way, including paying Vodka to Kuznetsov, he won't attack you even as Monolithian (as he's already nominally hostile).
* Make your way to the Agroprom through the direct transition from Cordon.
* Enter the Cave through the direct transition from Agroprom.
* Keep out of the Miner's chamber! Keep in mind that there are at least two mosquito zombies spawned for this quest. Navigate the cave to the Labyrinth entrance.
Once you get to the Monolith base, you will find Shadow of the Monolith on the main grounds. Talk to him and give him the Plasma Caterpillar and Rattle you brought. He will instruct you to go to the CNPP-2 and complete a job for his brother, leading to Brother of the Shadow of the Monolith.
=== Detour for Brother's quests ===
Find Brother and complete two quests for him: Stolen maps and PDA of the Monolith informer. Information on how to get there as a Monolithian can be found in the corresponding article. Once you obtain the code to the intermediate door from him, return to the Labyrinth.
=== Back to the Labyrinth ===
Go to the lower floor of the Monolith base, and enter the first code that Miner gave you: 434018.
Walk into the room and ascend the staircase. Be careful not to step onto a mine on the staircase:
Reach the second code-locked door and enter the reward code you received from Brother of the Shadow of the Monolith: 4241. The keypad is on the opposite side of the room.
Climb the wooden ladder and go through the corridor until you have the intersection with two ladders. The left ladder is blocked by a Burner anomaly and leads to a small room with the safe. You can manually trigger or jump over the Burner and grab the 20kg safe. Go back and climb the right ladder, which leads to a warehouse. In the far corner of the warehouse, there's a stash with your reward for PDA of the Monolith informer left by Brother of the Shadow of the Monolith.
Exit warehouse through the third door, entering the code 206018 you got from Fima the Charcoal during Captivity.
Now you've got one more thing to do: the Repair boxes for Sidorovich. Grab them before you exit the Labyrinth or Sidorovich won't take your Monolith amulet back, and you'll be stuck as a Monolithian forever.
=== Sidorovich ===
You can't turn the safe in to Miner yet, as you're still a Monolithian and he will attack you - get to Sidorovich in any way you can. A handheld Teleport to the Cordon will do the trick. You can also exit the Labyrinth to Yantar or the Cave and then travel to Cordon on foot.
Talk to Sidorovich and give him the Repair boxes. He will take the amulet from you, expelling you from the Monolith faction and making everyone around friendly or neutral again.
Double-check your reputation with all factions at this point, making sure it didn't drop compared to what it's been before.
=== Finishing the quest ===
Return with the safe to Miner. He will continue trying to crack the safe while you hunt down Fang's killers, referencing Confront Gavr. If this quest is already completed, you can speak to him again immediately for Safe cracker.
== Notes ==
* After the quest, Shadow of the Monolith will remain in the Labyrinth until a certain point in a different storyline (Assassination of the Shadow of the Monolith).
** Thankfully, he does not try to attack you even when technically hostile.
* Despite the warning that you need a CNPP-2 transition, you don't actually need it. It's still useful though, as you get sent to CNPP-2 in a followup quest, and wasting a Monolith teleportation device for that seems excessive.
** The earliest transition (from the Monolith Control Bunker) is given for Trap for Beast.
== Bugs ==