The Black Archive, shown for the first time in ‘The Day Of The Doctor’, is mentioned on page 24 of ‘Defending The Earth: The UNIT sourcebook’. There it says it is situated at an undisclosed location in Central London. We now know that it is beneath the Tower of London.
A single security guard waits at the end of the tunnel to let visitors in but there could be more security checkpoints that we don’t see. It uses a lock and key rather than electronics, designed to keep the Doctor out (presumably to prevent the use of his sonic screwdriver).
The staff has their memories wiped at the end of every shift, so that a security guard that has worked there for 10 years believes that it is his first day. This is said to be done through a memory filter, implying that only specific memories are removed (so that they can continue to have private lives without fear of erasure).
This would only make sense if it is done to the staff responsible for protecting and maintaining the Black Archive, rather than UNIT scientists who come to study the devices stored there. Otherwise UNIT would never know what was stored there or be able to make any progress in studying the alien technology.
A key plot point is that the whole of the Tower of London (and not just the Black Archive) is TARDIS proof. The Doctor says that this is possible with alien technology and human stupidity.
This raises the possibility of other locations being impossible for a TARDIS to land in, not just on Earth but elsewhere in the universe. The perfect excuses to prevent players short circuiting the plot by simply materialising the TARDIS where you don’t want them to go.
The big question is how it does this. How do you stop the TARDIS from emerging from the vortex in a specific location? There must be some way to manipulate space-time or possibly find a way to make them a fixed point where the TARDIS didn’t appear.
Much of their security is to prevent the Doctor finding out about the Black Archive and getting in. Kate Stewart believes that the Doctor would not approve of their collection. Yet he is aware of certain things they do have, such as the Vortex manipulator and the Space-Time telegraph. All three incarnations of the Doctor (the War Doctor, 10th and 11th) confirm as much when Clara mentions it.
Either the Doctor isn’t as against UNIT having alien technology as they thought or they have something that he doesn’t know about that would make him very unhappy. What could be worse that the weapons of mass destruction and time travel device they already have?
Buried 20 feet beneath the Black Archive is a nuclear warhead, powerful enough to destroy the Archive, its contents and London. It was to be activated in the case of an alien incursion. Kate seems to do this manually but there maybe a way to do this remotely. It is unknown whether the 5 minute countdown is a default or if the time can be changed.
The contents of Archive are considered to be so dangerous with a Zygon estimating that with its contents they could conquer the world in a day. This gives us an idea of how powerful these items are, although UNIT scientists might not yet know how to operate them all.
Amongst their collection they do have some rather mundane items such as River Song’s shoes and Amy Pond’s windmill toy. It is hard to know why these would deem important enough to be placed in the Black Archive (or how they obtained footwear from the future).
The Vortex Manipulator is given as an example of something that not even their allies can know they have. It is unclear if Kate is talking about allies of UNIT or allies of Britain. They fear that if America got hold the Vortex Manipulator they would alter history (the implication being they would make themselves responsible for every great deed).
Since we know that there is an American branch of UNIT it is conceivable that the UK branch is keeping information from the rest of the organisation. One has to wonder what the consequences would be if they found out.
When Torchwood existed how did the two organisations deal with their shared desire to collect alien technology? Torchwood was dedicated to protecting the country but UNIT has a responsibility to the world. This could lead to some conflict between the two organisations which could be explored in an adventure.
Following the Battle of Canary Wharf did UNIT recover Torchwoods collection for the Black Archive? This could result in several UNIT themed adventures with PCs despatched to recover technology from other shutdown Torchwood facilities.
The Black Archive can be used in a variety of ways in your campaign. It can simply be a safe place for alien technology to go at the end of an adventure. The PCs can leave it to UNIT to clean up after them.
It can serve as a resource. If the PCs are in 21st century London and need some heavy fire power or advanced technology to take down the enemy they could requisition it from the Black Archive. UNIT may require a favour in return.
The PCs could work for the Black Archive, either as part of UNIT or freelance. They could be tasked with obtaining an alien artefact and ensuring it is safely transported to the Black Archive or they could be called in to study an new acquisition.
They could find themselves in opposition to the Black Archive, if they have alien technology that the PCs feel they shouldn’t have. This can lead to an elaborate heist as they try to overcome its security.
Enemy forces (alien or otherwise) could target the Black Archive. If they can get in and disable the nuclear warhead the whole world could be at stake. Are the PCs trapped on the inside or are they inside when the facility is captured?
In a worst case scenario the Black Archive could be haven or an ideal place to make a last stand if the city is overrun. The PCs would have to quickly work out what the technology does in order to defend themselves and possible repel the threat.
The destruction of the Black Archive (and London with it) can be a good way to establish how dire things are in a dystopian future. A massive crater, surrounded by ash and ruin, is a stark image. Can the PCs prevent this from happening?
The existence of the Black Archive can be something that the PCs discover during an adventure or something they retro-actively knew about. They might also find that they’ve been there before but had their memory filtered.