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Mariposa Military Base

[...] based on the fact that these mutants have a super high concentration of the virus, it stands to reason that there could be some place which is creating them. As to where they could be, I cannot hazard a guess.Vree's autopsy report

The Mariposa Military Base is a former United States military facility built in central California to house Forced Evolutionary Virus research previously conducted at West Tek.[1]


  • In Fallout, Mariposa Military Base can be found thirteen squares west of Vault 13.
  • In Fallout 2, the base can be found ten squares east and twenty-seven squares south of Arroyo.


FEV Project

In 2077, the military built the facility in central California to continue research on the Pan-Immunity Virion initiative, subsequently referred to as the Forced Evolutionary Virus project, utilizing volunteer human subjects from the military as opposed to animals.[1] When the project was transferred in January 2077, the scientists who conducted the animal trials noted their disapproval regarding utilization of human subjects.[2]

Robert Spindel and Roger Maxson led the security team at the facility, but were not aware of the nature of work conducted there.[3][4][5] Once the nature of the research was discovered by the security team, Spindel suffered a nervous breakdown and Maxson took charge, attempting to handle the situation.[5] He ordered the scientists to be interrogated instead of executed.[6]

Chief researcher Robert Anderson explained that the testing at the facility was ordered by the government, as he outlined the experiments to the captain.[7] Maxson refused to believe him, and the scientist stressed that he was just following orders and that he was also a military man.[7] In response to the scientist's increased beligerance, Captain Maxson killed him.[7] Colonel Spindel took his own life and subsequent scientist interrogations ended in executions.[8][9] On October 20, 2077, Captain Maxson declared his unit in full secession from the United States, attempting to force the government to respond to the situation at Mariposa. No response came. Two days later, he ordered the families of soldiers under his command to take shelter within the facility.[10]

On October 23, 2077, the Great War struck. On October 24, Maxson ordered his soldiers and their families to prepare to vacate the base.[11] Sergeant Platner volunteered to take atmospheric readings outside the base and reported no significant amounts of radiation in the atmosphere, resulting in final preparations for the exodus.[12] On October 26, Maxson ordered the remains of the scientists to be buried in the wastes outside the base. A day later, on October 27, the group left the base under the leadership of Maxson, heading for the Lost Hills government bunker to the south.[13]

The Unity

The base was left sealed, but over the years, it slowly fell into disrepair, and eventually, various creatures made their way inside, mutating due to FEV exposure and began attacking settlements and caravans. The problem escalated to the point when Harold, a caravan master from the Hub, mounted an expedition to locate and destroy the source of mutants along with Richard Grey. The group failed to account for the automated defenses, which were still online.

Richard Grey's audio diary is found in the Mariposa Military Base, which describes a first hand account of mutation after he was knocked into vats of FEV by a crane.[14] He describes the green slime that he was immersed in and that it is the source of all the mutations his group traced to here.[15] He wrote that at first, his skin peeled and festered, and the experience was painful.[16] He recounted how he regained his ability walk and felt stronger, and described the experience of a tendril growing from his stomach which he used to consume other creatures.[17]

Grey commented that he had previously thought his mind was slipping into hallucinations, but now was improving.[18] In an attempt to see if animals could also gain awareness from the virus, Grey dipped a rat and a dog into the vats, and they became fused together.[19] At this point, Grey stated he was "much more than a human now" and began commenting on his plans to "bring others into the glory that is the Unity."[19][20]

Once Grey developed a neurolink into the military base's computer, he learned that the virus was called FEV and began to intentionally modify himself by way of injecting the virus into his body.[21] He dipped humans that he caught into the vats, but described them as disappointments, the virus mutating them into "big and dumb mutants."[22] When Grey caught and mutated a human that had less radiation damage, he noted that the conversion was more successful.[22] He then sought to expand to a new "unification center" in order to continue his vision.[23]

Grey subsequently began calling himself the Master and sought out radiation-free humans to transform into Super Mutants using what he referred to as FEV-2.[24][25] One of the Master's direct reports, the Lieutenant, explained that the virus itself was mutated by radiation after the war, and that humans living in the wasteland have been exposed to this mutant FEV, inoculating them from the full effects.[26] Individuals such as those living in vaults have limited exposure to this mutated virus and are still susceptible to its effects.[26] In addition to dipping subjects in the vats, some were experimented upon with FEV, the virus injected into various parts of the brain.[27]

The base was entrusted to the Lieutenant. Super mutants roamed the halls, restocking the armories and medical bays and soon it became the staging area for their conquest of the wasteland, as well as the only super mutant production center.[28]

Enclave's excavations

The Enclave Chemical Corps under Charles Curling isolated the most toxic elements of the virus found at Mariposa.[29] An excavation team was sent to the location, shielded by protective suits, to find a supply of the virus.[30] The team collected samples of the air and soil, both of which were contaminated by the virus.[30] Local inhabitants were recruited to dig down into the facility, many of which began showing signs of FEV exposure.[30] Super mutants remained in the base, forming under the leadership of Melchior.



Square footage: 30,000 ft
Before destruction...
...and after

Mariposa was a reinforced military base, constructed in the middle of nowhere, in the side of a mountain. The exterior had a characteristic white lining and a massive air conditioning vent, which provided fresh air for the complex. Inside, it consisted of four levels, stretching across a mile of depth.

Stronghold Level 1

Fo1 Mariposa Stronghold 1.png
Square footage: 40,000 ft

The first level of Mariposa housed the gear needed for any excursions, as well as a staging area for patrols. The main control computers for the base's force fields and robotic guardians were located here, allowing any infiltrator to considerably sabotage the base, just by altering the pest control setting for the robobrains, in addition to altering the force field status remotely. Apart from that, guard barracks were located on this level, for off-duty surface security teams.

Stronghold Level 2

Fo1 Mariposa Stronghold 2.png
Square footage: 60,000 ft

The primary soldier living quarters, medbay and armory were located on this level. The quarters were spacious, although spartan in design, with a large stockpile of weapons just around the corner and a well-equipped sickbay, stocked chock full of medical chems, surgical equipment and medical beds with integrated life support systems.

Stronghold Level 3

Fo1 Mariposa Vats 1.png
Square footage: 80,000 ft

The third level was divided in two, one was the general access area, where the assembly room and elevator to the commander's office were located, the second a military prison with an elevator going directly to the vats. When Mariposa was operating under government jurisdiction, experimenting with FEV, only scientists and their security staff were allowed into the prison area and the vats; soldiers under Colonel Spindel and Roger Maxson were barred from entry.


Fo1 Mariposa Vats 2.png
Square footage: 60,000 ft

Divided in two as well, the deepest level housed the massive FEV vats, together with their control room, scientist living quarters as well as the base commander's office. As with the third level, access to the FEV research area was strictly limited, with only scientists being allowed to enter. A special containment squad, the super mutant Vat Team 9, was stationed in the room near the cell block access elevator, to deal with any potential disturbances. It was composed of three soldiers.

The vats control room was where the base's mainframe was also located, holding personnel logs (Boyarsky, Roger Maxson, Richard Grey etc.) as well as the self-destruction initiation subsystem. Encrypted behind a firewall, these were nonetheless accessible for a skilled hacker. Alternatively, just blowing the console up would trigger the self-destruct sequence, but the base would be set to alert status, powering up the force fields.


Mariposa Encryption Codes
17652-1028-1285362 - 1 minute silent self destruct sequence
67209-5574-7805564 - 30 second self destruct, set base to alert
55658-8678-1251533 - set base to alert status
31914-1041-1251514 - 300 second silent self destruct sequence
53478-2565-7763929 - 300 second self destruct sequence, set base to alert


Fo2 Mariposa Entrance.png

The layout of the base is very similar to that in Fallout, although there are some tents (in one of them is the Military base holodisk 5) outside the base, wolf packs and the entrance to the base is blocked by rubble. To get in the base the player character will need to find the metal pole and put it on the rail cart ("Use on"). Next, they need to put some dynamite (can be found in the little shack right from the entrance to the base) on the rail cart (right-click and hold on the cart, and then choose the "bag" option; after that, simply click on the dynamite from the menu) and push it down. Doing this will net the player character 5,000 XP.


Stronghold Level 1

Fo2 Mariposa Level 1.png

When the player character arrives at the first level of the base, the first thing they would notice are the Enclave soldiers and mutant bodies lying around. The player character can loot these for holodisks and a plasma pistol.

The level is mostly filled with mutated rats (which can poison the player character if they bite), which can be easily dispatched. However, in one of the halls, there is a super mutant, Grundel, who will attack the player character if they try to talk to him. He is wearing spiked knuckles.

In the southwest corner (in the hall leading to the elevator), there is a broken power generator, which can be repaired for 1,500 XP (this can be easily done if one has Vic as a companion). Doing this will also turn on the lights in the base.


Stronghold Level 2

Fo2 Mariposa Level 2.png

The second level of the base is filled with Super Mutants, about 15 of them, but for an experienced player character, they shouldn't pose a large threat. Mutants are usually wielding power fists and flamers, although few of them use laser rifles and plasma rifles. In most of the lockers, the player character can find money, stimpaks, some ammunition and in the east corner of the map, a suit of T-51b power armor.


Stronghold Level 3

Fo2 Mariposa Level 3.png

Like on the second level of the base, the third level is also inhabited by super mutants that use laser rifles, plasma rifles, and flamers. Also, there are some pig rats, which can poison the player character if they bite them. There are super duper mutants on this level, which have greater HP than "normal" mutants. As comes to the loot, the player character should pay attention to the first room after exiting the elevator. In the room is the locker, in which the player character can find a green memory module, which can be used to expand ACE's medical library, thus allowing the player character to increase their Perception by 1. Finally, in the southeastern corner of the map, there is an elevator that takes the player character to the final level of the base, the vats.



Fo2 Mariposa Level 4.png

The deepest level of the base houses one of the tougher enemies in Fallout 2, Melchior. He is standing in the northwestern corner of the map, surrounded by goo which has remained there since the Vault Dweller destroyed the FEV vats.

Melchior himself is not that tough, but his abilities are unique. From the vats (or more precisely, the goo), he can and will summon Deathclaws, fire geckos, floaters and mutated mole rats. They are summoned in that descending order, and if the player character is strong enough to survive several waves of them, they are rewarded with some funny lines. He can have 4 summoned allies at a time, which can be bothersome, so it is best to dispatch him first, and then his "pets." This is quite challenging, even for experienced player characters with a good party, so they should be very cautious and prepared to leave the scene of battle with 1-2 fewer companions. Melchior also guards a locker containing a PPK12 Gauss pistol.



The background music is Vats of Goo, which was originally used for the intro in Fallout. The music is the same in both Fallout and Fallout 2 and is used for Broken Hills as well.


The Mariposa Military Base appears in Fallout and Fallout 2. It is mentioned in Fallout 3,[3][31] Fallout: New Vegas,[32] its add-on Dead Money,[33][34] Fallout 76,[35] and its One Wasteland For All[36] and Steel Dawn[37] updates.

Behind the scenes

According to Chris Taylor, the Mariposa Military Base is based on Fort Ord, a defunct military base in the Monterey Bay, California region.[Doc 1]


  • In a few cases the yellow force fields' device cannot be deactivated by the use of a tool. If that happens it's only necessary to simply use the Repair skill directly into the force field device. [verified]
  • In Fallout, in Mariposa level 2, there are some super mutants locked in a room. Loading a previous save will result in their death, or make them disappear entirely. [verified]

Related holodisks


Fallout 2


Сoncept art

See also


  1. 1.0 1.1 FEV research
  2. FEV Experiment Disk, Log Date January 7, 2077
  3. 3.0 3.1 Citadel terminal entries; Maxson Archive terminal, Maxson Family Dossiers, Roger Maxson
  4. Vault Dweller: "{207}{}{Base Information}"
    ZAX: "{242}{}{In light of significant advances in 2076 by the NBC on the Pan-Immunity Virion Project, the United States Defense Department, in fear of international espionage,}"
    "{243}{}{ moved a team onto the site to secure and oversee the project, now dubbed the FEV (Forced Evolutionary Virus) project.}"
    (ZAX's dialogue)
  5. 5.0 5.1 Captain Maxson's diary, October 10, 2077
  6. Captain Maxson's diary, October 12, 2077
  7. 7.0 7.1 7.2 Captain Maxson's diary, October 13, 2077
  8. Captain Maxson's diary, October 15, 2077
  9. Captain Maxson's diary, October 18, 2077
  10. Captain Maxson's diary, October 20, 2077
  11. Maxson log
  12. Captain Maxson's diary, October 25, 2077
  13. Captain Maxson's diary, October 27, 2077
  14. Richard Grey's audio diary; 0005 - 0020
  15. Richard Grey's audio diary; 0028 - 0043
  16. Richard Grey's audio diary; 0000 - 0004
  17. Richard Grey's audio diary; 0058 - 0096
  18. Richard Grey's audio diary; 0097 - 0111
  19. 19.0 19.1 Richard Grey's audio diary; 0112 - 0134
  20. Richard Grey's audio diary; 0135 - 0139
  21. Richard Grey's audio diary; 0154 - 0172
  22. 22.0 22.1 Richard Grey's audio diary; 0199 - 0236
  23. Richard Grey's audio diary; 0237 - 0281
  24. Vault Dweller: "FEV"
    Lieutenant: "The Vats are where the FEV is stored, and where you normals become one of us. Of course, some die horrible, excruciatingly painful deaths, but well, those are the breaks."
    (Lieutenant's dialogue)
  25. Vault Dweller: "I happen to know that your mutants are sterile."
    Master: "Preposterous! The FEV-2 virus doesn't destroy the reproductive organs of those it mutates."
    (Master's dialogue)
  26. 26.0 26.1 Vault Dweller:"Why am I a prime human?"
    Lieutenant: "The FEV was mutated by the war radiation. Those living in this desolate wasteland have been exposed to this mutant FEV, essentially inoculating them from the full effects."
    Vault Dweller: "So?"
    Lieutenant: "So, you're from a Vault and have limited exposure to this mutated virus. The original FEV in the Vats should work quite well on you."
    (Lieutenant's dialogue)
  27. Mad scientist: "It's not going well. I've been injecting the FEV directly into the pineal gland, but I'm not getting the results I'd hoped for."
    Vault Dweller: "Why don't you try injecting it directly into the Amygdala?"
    Vault Dweller: "Why don't you try injecting it into the Medula?"
    (Mad scientist's dialogue)
  28. Vault Dweller: "{125}{}{What is it you do here?}"
    Lieutenant: "{129}{LIEUT7}{My dear human, this is the great procreator! Here we'll make others of the master race and insure the Unity. It's all quite glorious, I assure you.}"
    Vault Dweller: "{131}{}{Sounds like it. Why do you need all this to, uh, procreate?}"
    Lieutenant: "{133}{LIEUT8}{Presently, there is a slight . . . problem in the reproductive process. It's being attended to. For now, we must use the Vats to turn humans into Super Mutants. You'll experience that glory yourself momentarily.}"
    (Lieutenant's dialogue)
  29. The Project
  30. 30.0 30.1 30.2 Military base level 4
  31. Harold: "It was a long time ago... I tend to lose track. I was exploring some sort of a military base with some other people... I think it was called Mariposa. We were pretty deep inside and we found some weird vats of this nasty green goo. Right when we were about to leave, I think we were attacked. Last thing I remember before blacking out was something knocking my friend into the stuff."
    (Harold's dialogue)
  32. Fallout: New Vegas loading screens: "Super Mutants in the Mojave Wasteland are remnants of the Master's Army and the Enclave's experiments at Mariposa Military Base."
  33. Courier: "When we were sneaking, you mentioned Lou... who's Lou?"
    Dog: "Lou... ten... ant? Long ago. Smart. Gave us all things to do. Like watch. And kill. Gone now. Miss Lou. Miss the Base. Miss the Church. Don't like here. Don't want to talk about it any more, makes head hurt."
    (Dog's dialogue)
  34. Courier: "Why you want to look like ground?"
    Dog: "When put it on, hard to see. Hard to see... Dog was told not to be seen then. Same as now, except different. Dog and others... had to hide in Base. And Church. The Master was there. Master was... spoke like many people. Smart. Like Lou-Lieutenant. Dog misses the Church. When got dark, no people, Dog could sit on floor and look up at ceiling. Could...take off wrist-collar then."
    (Dog's dialogue)
  35. Radio log: Aug 29 2077
  36. Watoga Towers terminal entries; terminal, lost count - message received
  37. Vault Dweller: "Where are you from?"
    Paladin Rahmani: "I'm originally from the California branch of the Brotherhood of Steel. I was posted near Mariposa before the bombs fell and I learned that some members of the US military had created an organization to help people. As a member of the US military, joining up with the Brotherhood was a natural transition for me."
    (Paladin Rahmani's dialogue)
Design Documents