So, the world experienced an apocalypse about 150 years ago. Sad times. Me and a couple buddies are the the salvaging business, and hello, recently we found a jackpot, an untouched military base. When we tried to raid it though....
The security measures are still capable of keeping us out. My question is, what security measures and equipment could keep out prospective scavengers upwards of 150 years after being abandoned?
I know you want proper booby traps, but I doubt they would be the biggest dangers or challenges of invading a military base. Security measures would have a far larger impact.
Firstly a 1 meter thick steel door should make it extremely hard to get in. With the electronics degraded and any ball bearings and grease long gone you will literally have to drag thousands of kilos of metal to get into the base. Then you have to basically break down every single door in your way. That's a lot of potential doors you need to bust down.
Elevators will also be broken. So if access to the secret underground bunker requires a lift, you're in for a tough time. You will need to scaffold down, break through the wreck of an elevator and pry open the doors. Depending on the depth this could basically be impossible, or require a lot of rope.
Oxygen. After 150 years, all the ventilation equipment will be broken. As you enter deeper into the base, you will start to run out of air. Either because the dying inhabitants inside have used it all up, or because you use it as you go in, and when you are trying to get out, it's still the same old stale air you breathed before. Depending on the size of the pathways, corridors and rooms, some sections can become death traps because the air won't move much, and will have low oxygen content as everyone has been breathing and passing through the area.
Unstable ground. While we would like to imagine that everything is still in tip-top shape, parts of the ground may have weakened. Metal stairwells, wooden boards. Maybe even the concrete floor, assuming some potential mishaps have taken place during the apocalypse. There is also a chance of water draining into the base, and since any pumps will have stopped working, it may fill up the lower floors, or wear away at the materials.
I'm thinking in an entirely different direction. In one of the Indiana Jones movies he met this cult that was dedicated to protecting a relic and keeping unworthy people from getting it. Suppose that, since the apocalypse, the locals have built up a cult that just absolutely does not want anybody to have what's in the military base. They could be the descendants of the security force that originally protected the base. It could sort of be a "Cargo Cult" in reverse. They build all the structures, follow all the rules, wear all the insignia, with the intent of preventing the return of the military.
So they've been busy doing things like digging the tunnels out to direct you the wrong way, while covering up and disguising the real tunnels. Or loading up a bunch of rusted out old junk in easy to get places so you think you've got nothing but trash. Meanwhile, whatever the real treasure is, they've got hidden behind some trick walls. Or buried under their village huts.
Previous answers have correctly pointed out that infrastructure and electronics decay over time. However, what if that particular base had been the test site for cutting edge military weaponry?
More specifically, AI research, with integrated combat and repair drones. As the world succumbed to the apocalyptic event which wiped out civilization as we know it, the AI received orders to seal the facility, and await further instructions from the military chain of command.
For 150 years the AI has been maintaining the underground facility, scavenging non-essential parts and pieces in order to keep repairs to the nuclear reactor and the basic infrastructure it requires to survive. It used on-site manufacturing capabilities to replace chips, hard-drives, etc., but of course its performance has degraded slightly over time. Maybe it's even gone a little insane.
When you breach the doors to the facility, self-defense conditions are triggered, which now allow it to venture forth into the world, gathering the raw materials and supplies it needs to replenish its arsenal.
A scorched-earth policy is a military strategy that aims to destroy anything that might be useful to the enemy while it is advancing through or withdrawing from a location. Any assets that could be used by the enemy may be targeted, for example food sources, water supplies, transportation, communications, industrial resources, and even the local people themselves.
On abandoning the base, its owners implemented their scorched earth defense. Radioactive waste was pushed thru the ventilation system. 150 years later the radiation is still strong enough to push your detector needles hard against the right stop. And you are lucky to have working detectors.
The ventilation system no longer works and that is another unplanned aspect of this tightly closed base keeping you out. The air is not breathable. Besides considerable radon from breakdown of the aforementioned waste, there is a lot of methane (leaking up from the deeper earth) and almost no oxygen (the methane ate it).
You canary will die from the bad air before it dies from the radiation. If you have solid radiation suits with onboard air supplies you could go around in there. Good luck with finding those 150 years after the fall.
The military base was in the process of being enclosed in a minefield. There is decaying explosives everywhere. No longer is there a need for triggers and proper arming of the devices, chemistry and time has made them into Russian Roulette To Go.
As the base was in the process of being enclosed in said minefield, not only are there mines scattered around, but also crates of them rotting all over the place threatening the intrepid explorer (intrepid because some stooges go first) with death AND destruction...
In europe 75-year-old unexploded bombs are found on a weekly basis. Once or twice a year one will explode on its own, sometimes creating a 30-foot-wide crater. Thankfully, usually in farmland rather than city centres!
Some of the bombs used in WW2 were chemical time-delay bombs, designed to explode after the initial bombing raid, to kill rescuers. Ain't war grand? Some of these worked by breaking a vial of acetone on impact, which gradually dissolved a celluloid sheet to release the trigger after a few hours. But if the bomb landed upside-down, instead of the acetone landing on the celluloid sheet it would gradually evaporate, with only the vapour weakening the celluloid. So instead of exploding with a 75-minute delay, they explode with a 75-year delay.
So it's plausible that bombs can still work - and sometimes become even more unstable - after decades.
(Of course, most bombs are rusted up and don't explode even when hit by a backhoe or a farmer's plough.)
Combined with the answers by bukwyrm and Willk, perhaps mines and tripwires and suchlike were set - either while abandoning the base, or by defenders who barricaded themselves in but have long since died.
And it needn't be all one sort of trap or bomb - perhaps your base's soldiers saw a lot of different IEDs in Iraq.
There are plenty of places where military bases are guarded, in part, by dogs. Sometimes, those dogs are real nasty:
Also, they're huge.
If a bunch of these got loose when the base shut down, and no one bothered to corral them, there's a solid chance a pack of them could grow and expand in the environs of the military base. A large pack of 30-40 feral guard dogs will be all the disincentive anyone needs to drop by the base for a visit.
Electronics and chemistry do not hold well against time, and electronics need also power to be operating. Therefore I don't expect them to be effective after 150 years.
Booby traps working with gravity will probably be still effective.
Something like:
requires nothing too complex to be hold and triggered. The only unknown aspect being wildlife accessing the building and triggering the traps.
Lots of stuff about passive defence here, not much about active.
Passive stuff is easy: fail-closed electro-mechanical systems are old hat, and what you can't conveniently lock you can always (conveniently or not) conceal.
But how about active deterrents that won't suffer from decay if they happen to sit idle for a couple centuries? Seems like we should have a lot more examples of those to choose from.
Unfortunately it's going to depend on how many people survived the apocalypse, how long they had and how badly they wanted to protect the base. If the answer to all three is 'enough' then we're in business.
Firstly, let's dig out all those old movies about trapped temples and so on. Get the engineering boys working on ideas for moving floors, pit traps, spike drops, you name it.
Next let's look for ways we can store energy that won't fade over time so that we've got power when we really need it. Chemicals, radioactives and so on aren't going to be reliable for really long time spans, but there's always the overlooked power storage: gravitational potential energy. Put some weights on chains hooked up to the most robust generation methods you've got, seal them in and fill the room with argon or something to make sure they don't get disturbed by rodents 50 years in. Rig it so that some mechanical trigger will remove the support from the weights and boom, you've got power. For a couple minutes anyway. Maybe long enough to get the fire started in your boiler?
Then there's the good old hydraulic method, where you have some hidden sluiceway from the local lake that can be opened to deliver water to a wheel that drives... well, whatever you need it to. Big chunky gears, collars instead of bearings, etc. Doesn't have to be efficient, just robust.
Or how about dropping weights onto piezoelectric crystals? That can generate fairly good electrical sparks. I reckon you could harness that to fire a block of plastique or some other electrically-fired explosive. It would have to be sealed into a chemically inert container to last long enough, but that should be easy enough to do.
There's always power if you look hard enough, and gravity is probably going to be the one that outlasts all the others. In a thousand years the output of your Americium-powered RTG might not be enough to do anything useful, but gravity is still going to be there. Just make sure that your mechanisms are simple and robust and sealed away from contaminants and you'll be fine.
And don't forget to leave a note for the scavengers to find - when they finally make it through your death traps - that apologizes for the fact that you didn't actually leave anything worth finding. Most of it got disassembled for raw materials to make all those traps. Including the one that just sealed off the entrance to this room and the one that's knocking all the supports out from the floor... hey, where did everybody go?
Yeah. I'd make a really nasty GM.
A story I read used an RTG to keep capacitors charged. Those capacitors energized various electric-fence-like obstacles, including strips across the floors of entry chambers to bunkers. The scavengers in the story were familiar with this design, and knew to short them before entering, but this wasn't common knowledge among others.
Very simply It is a nuclear hardened bunker and the door is locked, or worse rusted shut.
Hope you have a few months to hang around cutting it open.
Worse when you finally open it there is gaping hole, the supports for the floor having long since rusted away and collapses (not uncommon in old bunkers) and across the gaping hole you see... another *******ing door.
Sometimes the best protection is just time breaking some things and making other more solid.
Today about the most precious thing is data. And it is likely to be even so in the future.
So your hypothetical scavenger 150 years from now probably does not want to salvage rusty metals, and old computer hardware, but want the data. We are already very good at encrypting data, and not so adept at preserving it for a long time, so it will not be easy to salvage anything.
Maybe the military base will not be in real space, but a database in cyberspace. Some classified and encrypted data, preserved for eternity, and may worth a fortune for data collectors in the future. If only they knew the password ...
So from this, electronics should still work (at least anything not requiring batteries) and anything movable should work, as long as it doesn't require a lot of maintenance. The rest is up to you.
The other answers are likely to be what you're really after, but the first Emperor of China's burial tomb has lasted thousands of years without being breached. Sure, these days we don't dig it up for fear of contaminating it, but even as recently as a couple of hundred years ago we weren't so careful about such things.
My point is, it's somewhat hidden in plain sight, it's actually a logistically difficult job to break into (even by today's standards), and even if we got all the earth off the top, there's no guarantee that we'd be able to break into it without having to destroy some rock walls and the like. In other words, our investment into the task would probably out-weigh the gains we'd made by getting into it (even if it were full of 150 years old military goodies).
Last point: since technology of almost any type degrades over time, is it likely that 150 years old military kit would actually work? Most explosives lose their potency and/or stability over time, so weapons may not fire, bombs may go off without warning, etc. Vehicles or other machines would likely not work either. Maybe the whole lot has got a bit wet and is now a rusty pile of junk? Whatever you decide about all of that, it has a bearing on the "work versus reward" of breaking into the base in the first place.
Under the floor tiles, and strewn among debris, and buried in the soil, the defenders have left you here and there a sealed glass ampoule of some delightful relic of 21st-century life in an inert noble gas atmosphere. Sarin, Novichok, smallpox ... so many possibilities! Death never dies.
Honestly though, there are enough areas of the world kept uninhabitable by old minefields, that I have a hard time believing they would need to go to those extremes anyway.
