I think D6 is either a shelter or a weapon storage built by the goverment to hide important people from the nuclear, and the Dark Ones are their experiment to adapt human into radiated environment. But it went wrong and they are turned to a new race.
still, I don't think the Dark Ones are actually the "last light of hope", there must be a catch to this and there is a possible consequence in the future.
anyway, in Redux, they changed the Dark Ones from ♥♥♥♥ sapien to ♥♥♥♥ novus. it's possible that human will be wipe out so the Dark Ones will become the new "human" of Earth

That's pretty close, except "Homonovus" is straight from the first scene in the original 2033 with Alex and Hunter (who still lived after the events of 2033, fun fact). D6 never had any supplies other than weapons (they state that in game dialogue), that is why Artyom implies that D6 would curse the Metro inhabitants. Even after the world is over the guns of their fathers are still damning them. D6 is definitely a shelter but more for weapons, tanks, and missiles than people. It's designers had a crap hashed plan to use those supplies to retaliate against their nuclear aggressors. Of course they were too naive to think they could just recover if they could survive the blast. D6 is part of the Metro 2 lines (that actually exist below public subways in Moscow).

The "invisible watchers" (another human faction) are said to inhabit the Metro 2 lines and still have a way to the University as well as other hidden entrances to the main Metro tunnels. Polis is made of 4 factions and one of those is the elder Brahmins that make up the last scientific minds from the old world left in the Metro. The invisible watchers abandoned D6 shortly after retreating underground because it was all self destructive weapons and they are said to be able to communicate with the Brahmin leaders at D6. Except the invisible watchers have been struggling themselves, but they can't expose themselves, risking the chaos that is the rest of the Metro spreading to them. The schism among Polis factions and Hunter's words give the implication that the Brahmins revealed certain secret information to a privileged few. Namely, Hunter was told about D6 and the Dark Ones long before the beginning of 2033 with Artyom, as he refers to the Dark Ones as "old friends" on the tape he leaves to Red at the Ranger hideout in Dead City.

I think the knowledge given by the invisible watchers about the Dark Ones and D6 is the primary reason that the Polis council (made up of th 4 Polis factions) denied Miller's expedition there. Indicating that Hunter was privy to more info than Miller, which could explain how he fell from grace in the 2034 novel. When he defies them and occupies D6 anyways, you'll notice at the start of LL it is really only Rangers there, even the merchant is a Ranger. The Brahmins disdained the Spartan occupation because of their prior knowledge of D6 and their perpetual use of violence to solve problems. A journal entry reveals Artyom's speculation about the origin of the Dark Ones actually being a prewar experimental soldier that feeds off radiation and can communicate without equipment, think hivemind. This is how the invisible watchers and the Brahmin could know about the Dark Ones stashed away in D6.

that's a great theory.
I thought it was hinted in Last Light that the Invisible Watchers are actually the Dark Ones. but to think of it again, it could be a survived government faction. I believe there wasn't much mention about them (the government) since the book so I guess the sequel will focus on that plot, either that or the true origin of the Dark Ones.
if the Invisible Watchers are acually the goverment, it's possible that Korbut is actually one of them (or involved with), and was fooled to attack D6 so they could get rid of D6 and the Spartan at the same time. if "good ending" is canon, then their plan has failed, and this is where they have to come of out hiding and do their own dirty work.
Last Light's "good ending" pretty much cleared everything about the plot, but there is still an unknown future waiting for human race

Ya, and then there is Khan, that is an even deeper conundrum. Artyom also suspects that librarians were once human.

The Dark ones may have been intended to work for the survivors on the surface, or serve as soldiers. I do not see the survivors wanting their heirs somehow converted to Dark ones to live on a contaminated surface.

The idea of demons, nosillas, shrimp and other large creatures evolving or mutating in large numbers in the few years it takes Artyom to grow into the child who opened the gates outside is really shaky, likewise the demons and other monstrous creatures.

The poison gas is concentrated in pockets that make no sense 20 years later, it would make more sense to have the creatures arrive evolved from dimensional rifts from some physics experiment interrupted by the war. The portals were fully opened long enough for large animals to come through then closed so that mostly atmosphere from the other side leaks through in places. The violent storms of the past 20 years would have dispersed the gases long ago and how are there such battered buildings that are so air tight?
Check out the movie “The Mist” based on the Stephen King novel.

All I know is this game needs a sequal.

I would like the sequel to be soon, I believe this franchise has the potential to be for it's genre what the Mass Effect franchise is to it's genre.

But it needs to button up the glaring inconsistancies about concentrations of poison gas not being blown away by 20 years of the harsh and penetrating winter winds of the Russian winter and it's blizzards.

What is generationg the same poisonous gases so consistantly over such a large area?

How did so many species that thrive in these gases mutate and spread so far and wide so fast in such numbers?

The developers have the talent and skill to create quality characters, their criteria for the good ending seems a bit contrived and imposed by outsiders social engineering the hearts and minds of the gamers, (penalize you for fighting your way out of a room full of killers for the loot you need to survive.)

I doubt even Mass Effect could shrug off not having better explainations for something as big as all the mutants and the poisinous gases.

I always assumed the poisend air outside wasn't toxic gas, but simply air chocked full of radioactive fallout. A mere 20 years after the war and you only need a gas mask to be outside, a real nuclear war wouldn't be so kind, and I dare say even in underground metro's you wouldnt be safe either, as they are not airtight.

Originally posted by BlackWater:

I always assumed the poisend air outside wasn't toxic gas, but simply air chocked full of radioactive fallout. A mere 20 years after the war and you only need a gas mask to be outside, a real nuclear war wouldn't be so kind, and I dare say even in underground metro's you wouldnt be safe either, as they are not airtight.

Particulate fallout would be like smoke, pretty evenly dispersed and even the finest particles would precipitate out in three or four years like the fine ash of a massive volcanic explosion, the writers got the idea from sci-fi writers who took artistic liberties with the facts of NBC warfare. You could kick up fall out particles the settled to the ground years ago, but ckeck the reality of nuclear fall out.

Fallout protection is almost exclusively concerned with protection from radiation. Radiation from fallout is encountered in the forms of alpha, beta, and gamma radiation, and as ordinary clothing affords protection from alpha and beta radiation,[12] most fallout protection measures deal with reducing exposure to gamma radiation.[13] For the purposes of radiation shielding, many materials have a characteristic halving thickness: the thickness of a layer of a material sufficient to reduce gamma radiation exposure by 50%. Halving thicknesses of common materials include: 1 cm (0.4 inch) of lead, 6 cm (2.4 inches) of concrete, 9 cm (3.6 inches) of packed earth or 150 m (500 ft) of air. When multiple thicknesses are built, the shielding multiplies. A practical fallout shield is ten halving-thicknesses of a given material, such as 90 cm (36 inches) of packed earth, which reduces gamma ray exposure by approximately 1024 times (210).[14][15] A shelter built with these materials for the purposes of fallout protection is known as a fallout shelter.

The danger of radiation from fallout also decreases with time, as radioactivity decays exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. For example, after 7 hours, the average dose rate is reduced by a factor of ten; after 49 hours, it is reduced by a further factor of ten (to 1/100th); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.[15]

You make a good observation about the spread of contamination through all structures that have not been carefully maintained over 20 years.

Leakge of gasses and life forms from another dimension are the easiest sci-fi explanation for these anomilies I can come up with. Google nuclear fallout, Nagasaki and Hiroshima, you will understand why Hiroshima and Nagasaki are now the prosperous modern cities they are,

11. Hevesi, Dennis. "Dr. Louise Reiss, Who Helped Ban Atomic Testing, Dies at 90", The New York Times, January 10, 2011.
12. Kearny, Cresson H (1986). Nuclear War Survival Skills. Oak Ridge, TN: Oak Ridge13. National Laboratory. p. 44. ISBN 0-942487-01-X.
Kearny, Cresson H (1986). Nuclear War Survival Skills. Oak Ridge, TN: Oak Ridge National Laboratory. p. 131. ISBN 0-942487-01-X.
13. "Halving-thickness for various materials". "The Compass DeRose Guide to Emergency Preparedness - Hardened Shelters".
14. Kearny, Cresson H (1986). Nuclear War Survival Skills. Oak Ridge, TN: Oak Ridge National Laboratory. pp. 11–20. ISBN 0-942487-01-X.
15. Kearny, Cresson H (1986). Nuclear War Survival Skills. Oak Ridge, TN: Oak Ridge National Laboratory. pp. 11–20. ISBN 0-942487-01-X.

Of course it is fiction, just blame it on magic. Mass Effect has way more inconsistencies because they try to explain everything. Metro tends to leave things up to interpretation, make the consumer reflect on how the events in the book make them feel about real life.

Originally posted by Jig McGalliger:

Of course it is fiction, just blame it on magic. Mass Effect has way more inconsistencies because they try to explain everything. Metro tends to leave things up to interpretation, make the consumer reflect on how the events in the book make them feel about real life.

A discovery of records in a locked lab in D-6 or elsewhere could explain a lot, quick, cheap and easy. Discovery of resources to produce quality activated charcoal (that isn't that hard to produce) could make exploration a lot easier too.
You build a virtual second city under your capital and you do not build prduction facilities. These the people who moved most their critical factories beyond the Urals by train so Hitler's air force of medium bomber could not touch them. This is the country that disassembled and shipped to Russia the most valuable surviving production facilities of their conquered enemies; they appreciate means of production to meet their needs.
These people wouldn't store at least adequate machine tools to restart and rebuild their industries?
Repair the tech to seal the dimensional rifts, stop the flow of toxic atmosphere let what is there dissipate and precipitate out, maybe with the help of the Dark ones who don't do well for long in our atmosphere which is polluting their home world as theirs does ours, return to the surface and rebuild on a growing scale.
Do the books have anything that contradicts any of this?
The cold war has restarted; numerous countries seek their own nuclear arsenals, what happens to games that promote the myth, if with access to the internet the truth about nuclear fallout becomes common knowledge?
Isn't this like a game claiming Martian canals, cities and an ancient Martian civilization preparing to invade and make us slaves? Everything perpetuating misinformation decades out of date.

LOL @ reading my OP from 10 years ago, I was really invested in the story :)

Pretty sure I was on my second reading of the novels.