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There is a huge bank vault with billions worth in save deposit boxes (jewelry, bullion, documents, certificates, etc) and cash reserves. It is also a regional base for ATM cash resupply, so there are dozens of shipments of cash coming in and out every day. There is over a billion in cash at any time in this vault.

Unfortunately, bandits are planning to dig a tunnel into the vault's floor, and stealing the riches through the tunnel.

How can I stop them from robbing the vault, or at least get a fair warning when they do?

Also, our organization can procure most permits, but anything blatantly illegal is off-limits.

I only have standard, commercial early 21th century technology. This is , so please have some scientific backing. There is no need to cite published research, but won't hurt if you do.

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closed as off-topic by sphennings, MichaelK, Mołot, Vincent, Ash Oct 4 '17 at 15:16

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    $\begingroup$ It seems like you've already built your world and are asking us about a scenario that happens in it. $\endgroup$ – sphennings Oct 4 '17 at 14:32
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    $\begingroup$ Why are you asking this to be science-based worldbuilding when you are clearly looking for idea generation for a story? $\endgroup$ – MichaelK Oct 4 '17 at 14:33
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    $\begingroup$ I am sorry but you get what you give. If your entire premise is "Scrooge-like character, bandits in mine-carts, tunnel, ANSWER ME WITH SCIENCE!"... then you will not get any hard science back. Handwave in = Handwave out. Stand by for answer... $\endgroup$ – MichaelK Oct 4 '17 at 14:40
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    $\begingroup$ I'm unsure how modern vault security is about worldbuilding. $\endgroup$ – sphennings Oct 4 '17 at 14:40
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    $\begingroup$ No that did not improve it a lot. It is now less "funny" but still as vague. And it is still extremely scenario specific. And it is not helped by demanding that is be "science-based". $\endgroup$ – MichaelK Oct 4 '17 at 15:10
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A very simple way is by having a ‘floating’ vault.

Dig a large room, then build a second, watertight room inside it, suspended on a series of pillars.

Now route a stream or river into your outer room through some well secured gratings. Don’t tell anyone where it goes. Plus points if you use it as a power source.

Et voila: anyone attempting access by digging tunnels from below is going to have a Bad Day, and you’ll be able to monitor the water outlet to check if someone has provided another way for the water to escape.

ADDENDUM: I may have misread the century. This is still a valid defence, but now you can use pressurised water and more sophisticated construction methods.

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It is also a regional base for ATM cash resupply, so there are dozens of shipments of cash coming in and out every day.

This vault is going to be open most of the day. Just put permanent guards in the vault and the entire tunneling threat is gone.

Besides, in a vault that sees this much activity, "tunneling" is a really poor vector of attack anyway. You'd be better off impersonating one of the many people that go into the vault every day to collect money for ATMs or whatever.

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Generally speaking you can't do anything to stop them.

There are a number of things you can do to slow them down and most importantly to detect the tunneling so you can take other actions to stop them.

The most basic is detection, monitor the ground area around the vault for any seismic activity indicating digging or drilling. This is standard practice at borders to detect illegal tunneling and around vaults or other underground secure areas. There are a number of existing security solutions: Relevant Google Search. This is complicated if you are in a city or other environment that would have regular underground construction, so an isolated site would make this much more accurate, as well as making it harder to disguise the starting point for the tunneling.

In order to make the tunneling more difficult you can put the vault deeper, requiring more difficult tunneling to reach. At the extreme end, you could use drilling techniques developed from oil exploration and make a "vault" that is only a foot wide, but is several miles underground, and the contents can only be accessed via a long elevator like connection. This would limit the amount that can be stored, but it would make it really hard to get to.

In terms of making digging harder, you can site your vault in natural rock formations that would be harder to dig through (very hard rocks, multiple layers of different material, underground gas pockets, etc.) Or you could surround your vault with layers of complicating materials, like encircling the vault with layers of reinforced concrete, or pressurized water/oil bearing rock materials. These would make digging through them very difficult and are likely to allow detection when/if these outer layers are breached.

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Seismic sensors, thick concrete, plain old burglar alarm, big safe

Detecting digging is easy: you use seismic sensors and microphones.

Preventing them from digging right into your building is done by making the entire foundation and underground floors have sides and underside be thick concrete. Make it a couple of meters thick at least. Breaking through concrete is extremely noisy and takes lots of time. For added fanciness, embed channels in the concrete. If the channels break, like a wire running through them gets cut or they cannot maintain air pressure, then you know someone is trying to get through the concrete.

The vault itself is placed above and away from the outer walls, so that the burglars will have to step into the building to reach it. Inside this building you have plain old security measures, such as cameras, motion detectors etc. And you will of course have a security force ready to deal with detected intrusions.

The vault itself is made big and heavy. This way the burglars would have to crack open the vault and not just carry it with them in the tunnel. The size of the vault further delays them from getting into it.

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    $\begingroup$ If you're going to VTC a question it's poor form to answer it. $\endgroup$ – sphennings Oct 4 '17 at 15:18
  • $\begingroup$ @sphennings Is it now. $\endgroup$ – MichaelK Oct 4 '17 at 15:39
  • $\begingroup$ You can read here about why it's poor form. $\endgroup$ – sphennings Oct 4 '17 at 15:41

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