A long long time ago, the EMPRAH declared that humanity had to spread across the stars. He foresaw that humanity would need the resources of a galaxy to withstand the trials of the future: mad gods created by overindulgent jackholes, gun-toting walking foot-fungus infections, and emo robots with
plasma gauss weapons.
"But your eminence," asked some foolish minister, "only a fraction of 1% of our population has the skills and knowledge to survive outside our urban hives, never mind on worlds that haven't been terraformed yet!"
"Don't worry; I have a plan. For the past century I've been seeding book publishers with the idea of writing self-help books with names like '... For Dummies' and 'An Idiot's Guide to ...', and I invested in a manufacturing company in the Ikea Norse Hive when it was called 'Sweden'. Centuries later these and many other ideas will bear fruit in a push-button, turnkey, E-Z Bake oven colony procedure tool I will call "THE STANDARD TEMPLATE CONSTRUCT" and it will enable the most humble among us to reap the benefit of the stars!"
"... E-Z Bake oven, your highness?"
"Ah, an idea before your time. Actually not one of mine; clever little thing. I liked the frosting that came with the cake mixes."
Every colony-ship of humanity carried an STC with them, with easy-to-comprehend blueprints and instructions for everything that a colony would need, from how to use local resources to harvest food or irrigate deserts, to the architecture required to build mile-high hive arcologies or tap a planet's core for the magma pumps needed on forgeworlds. This also had the intended effect of making sure that manufactured goods were compatible between colonies across the Imperium of Man, and that the cultures would stay (relatively) the same and no colony would become so different as to seem as aliens to other humans. This is the reason why everyone across the Imperium speaks the same language, among other similarities between disparate colonies.
However, the STCs were created millennia ago. And there was a Dark Age between then and now. And some damn fool idiots decided to 'port the STCs to Windows 8 to make sure it would be "more compatible with the smaller devices colony ships will be carrying." As a result, no fully intact STCs have survived to the modern era. All that's left are the short passages that have been copied religiously from generation to generation in each colony, but only what that particular colony needs... if it hasn't been corrupted by thousands of years of transcription. Discovery of a new outpost of humanity is a prestigious discovery, as there's a chance that there may be parts of their STC that were preserved better than others, or even whole pages that are unseen elsewhere. Hoping beyond hope of course, there is still the possibility that an entire working, uncorrupted STC machine exists, probably on one of the worlds in the Halo Zone.
The STCs are a very important thing for the tech-priests of Mars, because they are a bunch of nutters who for the past ten thousand years have done next to nothing to promote technological progress because they think their machines are alive and that modifying and developing them will hurt their precious feelings. Finding an STC is thus the only possibility the Imperium has for restoring humanity to pre-technutter levels. If it manages that, it could surely wipe out all its enemies.
But of course, this being W40k, where the status quo is God, this will never happen.
 Revelations from recent STC recoveries
Scientific discovery after a dark age can be equal parts research and archeology, as a culture reclaims knowledge it has lost. The Adeptus Mechanicus is particularly interested in finding new STCs to fill in the gaps of their knowledge, to de-mystify parts of their manufacturing rituals, and to issue new and improved tools to the Imperium, and are willing to pay handsomely for even a fragment of an STC. Basically, if you find the STC for a better toaster you're set for life; find one for an entirely new weapons system and you'll probably end up being made governor of an entire planet.
Among the recent discoveries from recovered STCs:
- The Land Crawler all-terrain soil cultivator vehicle
- An artificial sweetener that doesn't have an aftertaste like foot sweat
- The Castigator-class Titan (whose STC turned daemonic)
- Cultural archives badly lost over the years but basically going back to before M0
- Jodorowsky's Dune (holy fuck watch it)
- The HXT-37 voltage transubstantiation cable for using 'AA' batteries in 'AAA' music players
- The 58th flavor of Heinz's ketchup
- Daemon-corrupted Men of Iron...err maybe this wasn't such a good find. Cue Ibram Gaunt BLAMMING said STC.
- A simple (durable, cheap, mono-molecular) combat knife. Two scouts found it and were rewarded with a planet each!
- Leman Russ Punisher and Eradicator (actually a case of "hey! Look at this Ancient technology I just... found. I didn't invent it at all, really")
- Various life extension methods like better juvenants, etc
- Helmets manly enough for space marine commanders
- Kamikaze neckbeard powered toaster ovens (Caestus Assault Ram)
- Baby's First Thunderhawk (affectionately called the "Stormraven Gunship" by some.)
- Super Baby-Carrier 40000 (incomplete STC fragment, but they still managed to build the Nemesis Dreadknight out of it...)
- Miracle chocolate that somehow only manages to go to women's lovely parts. Extensively eaten by Boone
- Panacea - see the sad story below
- Dreadclaw Drop Pods. Something wasn't right with these folks Machine Spirit. Maybe they really were alive?
Notice that most of the vehicles in this list did not exist until later editions of the game. This is how Games Workshop and Forge World justify adding units to armies (Imperial armies, anyway) -- they existed all along, it's just that the Adeptus Mechanicus only recently re-discovered (or "re-discovered" and most definitely did not invent) the STC pattern encoding them. Basically, you can know how to build the vehicle, but if you don't know what programs it uses or, possibly, what specific materials to use for more sensitive components, it isn't going to do jack.
Of course, any time that a real game-changing STC comes along, something bad happens and it gets stolen, lost, destroyed, or corrupted, because grimdark. For example, in the 5th edition Dark Eldar codex, Asdrubael Vect's Ex, Lady Malys, tricked some Orks into attacking an Industrial world that had been fortified beyond belief. Every hive on it was crammed with guardsmen, and even titan legions and Admech forces were present because they had found an STC called Panacea that would allow
billions trillions quadrillions the whole of humanity to be cured of disease and poison (hint: Panacea is a literal cure-all). The Dark Eldar attempted to steal it by luring an Ork fleet to the planet where the Orks pretty much crushed all resistance, allowing the Dark Eldar to effortlessly take the STC (after the Orks stole it first, go figure). Of course the Dark Eldar don't use the same technology (and could not care less about cures for human diseases anyway), so at the moment it's collecting dust in Malys' Trophy Room, rather than alleviating the grimdark. At this point, we can only hope that the Dark Eldar manage to provoke the Imperium into launching a full crusade against Commorragh and rescue the STC in the process... yeah, right. that did happen before this incident, though. Vect stole a fully manned Salamanders Strike Cruiser (but wasn't stupid enough to try and board it, for obvious reasons). The Salamanders responded by figuring out how to break into the webway, tracking down the lost/hidden city of Commorragh, and butchering the fuck out of all of Vect's rivals (whom he arranged for the Strike Cruiser to be near for this very reason). They then took their Strike Cruiser back and left much of Commorragh a smoking ruin. Something Malys probably overlooked (or didn't know, somehow) upon her acquisition of this STC is that the Salamanders still know how to get into the Webway and know where Commorragh is. They also happen to like killing Dark Eldar, and now the Mechanicus and the rest of the Imperium has a very good reason to launch a full (i.e. thousands of ships and millions of Guard regiments with dozens of Space Marine Chapters) Crusade against the point-eared bastards...
 Baron von EvilSatan's AdMech/STC copypasta
The Mechanicus does NOT have the technology. They haven't been living on some fancy paradise planet since pre-Fall. Mars is an anarchic nightmare shithole the moment you leave the safe zones into the kilometres of labyrinthine corridors beneath it full of rogue machinery, self-aware and malevolent AI from before the Fall, and the daemon programs of the Heresy. EVERYTHING in the databases is fucked. The databases are fragmented over the entire surface to the extent that it would be impossible to see one tenth of the total files in the ludicrously extended life of a Magos even assuming that they are completely safe to visit. And they are not.
The files have been corrupted into madness by the Fall, and the unleashing of the most potent informational warfare systems ever to exist to defeat the Iron Men. Nearly all of Mars was rendered uninhabitable, what they live in now is built on the top of the ruins. They send archeotech expeditions in to find shit, nearly all of them never come back. The sheer number of rogue war machine running around in there is sufficient to rape the mind. Then came the Heresy, which was not earth-exclusive. Mars as the second most critical planet in the Imperium was the site of fighting nearly as ferocious as on Terra, with Mechanicus loyalists and Hereteks fighting tooth, nail, and mechadendrite everywhere. Ancient machines were unleashed, viruses both normal and daemonic unleashed into all the computer systems. Nearly every single stored record on Mars was rendered unusable, and those that survived are half the time self-aware and don't like you, or daemonic and actively try to kill you.
If you come back with a schematic, it is almost certainly gibberish, and if it isn't, it's probably corrupted into uselessness. If it does come back whole it was probably malevolently fucked with so that instead of a Lasgun power cell it's a fucking grenade set to detonate the second you finish building it. Why do you think they want off-world STCs so damned much if they had them all here? The fucking Heresy is why. Off-world they only have to contend with the Fall's war and its effects on the machinery plus twenty thousand years of degradation with no maintenance. But at least off-world it'll probably just not work instead of actively seek to kill you.
Why do you think they seek to placate the Machine Spirit? It's because it exists. The fragments of trillions of self-aware programs, flourishing during the Dark Age of Technology and shattered by Man in his war with the Iron men, imprisoning the few who had not set themselves irrevocably into the machinery, a prison smashed wide open by the Heresy. Everything that can hold programming in the Imperium has a shard of a program in it. EVERYTHING. And you'd better fucking please it or it will do everything in its power to make your day shit. Sure, if it's a Lasgun it'll just not work or start shooting off rounds by itself, but if you piss off a Land Raider you can say bye-bye to half a continent. They apply these principles to things without spirits by habit, since they're so used to dealing with tanks that if not talked to just right might go rogue and annihilate the Manufactorum before they can be killed.
This is why they do not like ANYONE fucking with technology, because it is so rare to find anything that just works it is critical it not be compromised. That, and they do not have the actual knowledge to fuck with it intelligently, just through experimentation, which inevitably leads to slaughter. Pressing buttons to see what works is fine in a 21st century computer, but it is a very stupid thing to do at the helm of a 410th century starship with the destructive power to end solar systems. The entire knowledge base of humanity was lost. Not forgotten, but outright lost. Everything at all, poof. Nobody knows anything because the Fall fucked everything up and the Heresy double-fucked it. To rebuild the theoretical framework needed to design new technologies that don't kill everyone near them would require starting from the ground up. They don't have the time, and they never have.
This gets on to the point of war and what it does to technology. Someone will parrot that it makes it go much faster. Yes, it makes practical applications of technology go much faster. It also utterly stops all research on the scientific theories behind those technologies. This means that when war chugs along for a decade or two things get done. It means when it goes on too long you run out of theories to turn into technologies, and then you run out of technologies to apply. You stagnate. When you have been fighting in a war for survival in a drastically overextended empire, this is what happens. You are desperate for any extra materiel that can possibly be produced. Half your entire fucking military might went rogue, smashed the half that stayed, leaving you with the tattered shreds of a war machine to keep hold of an empire that was reaching straining point with an army far larger. There is no time for the sort of applied research programs that took Man twenty five thousand years to develop, in a time of unprecedented growth and prosperity.
This is also why the Adeptus Mechanicus insists on cargo cultism. It's because when you are dealing with things you barely understand because everything you knew about them was destroyed it is the safest and most reliable option. The rituals do not exists for mysticism, they exist because they are the most practical means of building, repairing and maintaining the equipment they have with the knowledge surviving. You don't understand why pressing that button makes it go, because the manual tried to take over your brain and the copies are all unreadable and the research base that would let you reverse-engineer it does not exist and cannot be built.
Why are the Tau doing so well with their technology? Because they had peace. Eight thousand years unmolested by any enemy and they were helped the entire time by the most advanced biological race in the galaxy. Give the Imperium eight thousand years of peace and I can guarantee you it will be harder than it was during the Great Crusade.
Since some still don't get the idea, try this.
Build a library, fill it with all human knowledge. You take it elsewhere when you need a book from it, but the book is only a simplified copy. You don't understand the real book, and you don't need to. Nobody takes the real books anywhere because why would you, when there's a whole library there?
Now that library goes rogue and the maintenance machinery starts killing everyone any-fucking-where near it. Where the fuck did they all come from, you swear to god there weren't this many, and there weren't because they're using the library's information to fight their war. The government fights a battle that destroys the planet against these robots and tears apart the library to stop them using it, only to be destroyed in the process. The library is leveled, cast into flames, every book burned and every computer virus-laden.
Then comes a man who worked there. He talks to the few surviving library workers, assembles their information, and starts rebuilding a city around the library and expanding it as the librarians find little scraps of paper and fragmented bits of files that stuck together just right read something. They rebuild a library from scrap on the ashes of the old. It isn't a shadow on the glory of the old, but it is all they have.
Then the city turns on itself, kills its master, and the librarians turn to rage. Half of them kill the other half and destroy the remnants of the library because where they're going they won't need science. Everything burns, and the city is left to a scattered few survivors, walls open to the world, with the hungry predators circling.
The Adeptus Mechanicus is the sole surviving librarian, desperately scrabbling through the ashes of paper and splinters of hard drives for anything to help him and the city he needs to survive just a second longer.
The Imperium isn't grim because things suck by choice and could be fine if a sensible person came along. That sensible person wouldn't survive fifty seconds of the reality. The Imperium is grim because every single shit decision, every single sacrifice, every single death, every single man, woman, and child suffering a shit life in the worst conditions imaginable, is the absolute best that can be done. It is a study of the worst happening to everyone and what part of your humanity must be sacrificed today just to stand a chance of survival, and all it asks is whether or not it would have perhaps been better to die.
++ The optimist believes this to be the best of all possible worlds;
The pessimist fears this to be true. ++
++ Exsisto iustus quod vereor non. ++
Ok, ok, you dont get it yet? You still think this is some sort of Star Wars/ Star Trek universe where the good guys can get what they want? Well, go check page 21 of Codex Daemons Sixth Edition, you get a planet named Thruscas Sine which decided to fuck-off with AdMech tradition and use awesome science to erradicate all natural diseases in just one single generation... Only for the inhabitants to get their world infested from pole to pole by Nurgle's armies, who took offense of such a feat, yeah, now you see? Can you feel the tears of rage in your cheeks? No matter how hard they try it, there is always a ways to get humanity fucked in Warhammer 40,000.
 The Ark Mechanicus
You are Ark Mechanicus. You are Speranza. You are the bringer of hope in this hopeless age.
- SPOILERS from Priests of Mars
In this novel written by Graham McNeill, it is revealed that the Ark Mechanicus Speranza, an incredibly old and massive ship used by the Adeptus Mechanicus to explore new stellar territories, has some of the most advanced technological achievements of mankind encoded in its very structure. This revelation, unfortunately, was only discovered during a brief moment when one of the main protagonists of the novel, Archmagos Lexell Kotov, made some sort of spiritual connection with the Machine Spirit of the Speranza(alignment: True Neutral) in order to save the day, and he forgot what he had seen immediately after. Which may suck, yes, but this was compensated by the fact that upon the Archmagos linking with the ship, the Speranza's AI went godmode, deploying all kind of unimaginably super-high-tech targeting systems that NOBODY knew it had, systems that were capable of functioning with 100% precision in the middle of a space-time gravitational storm, and detected and
crippled fatally damaged an eldar cruiser in ONE FUCKING SHOT using a dorsal mounted chrono-weapon so unbelievably advanced even the necrons would have been scratching their heads trying to understand how it worked. That's right, lads, the Ark Mechanicus ships which the Imperium already own and operate could be the answer to the missing information of the STCs and more. A shame not even an Archmagos can access the information without immediately forgetting it all once the interface with the Machine Spirit has been severed. Especially given that, once linked to the ship, Kotov realized that (perhaps all of) the Ark Mechanicus used by the Mechanicum are STCs.
Still. One-shotting an eldar cruiser with sniper-precision in the middle of a space-storm that should have made locating said vessel completely impossible, much less being able to fire at it with any hope of accuracy.
Humanity's old tech was scary-powerful. How the mighty have fallen, eh? From turbo fuckyou chrono-weapons to ineffectual flashlights. That's not even grimdark. That's just outright depressing.