Doomsday shelters might make you think of poky bunkers buried deep underground, but that couldn't be further from the truth for the ultra-wealthy.
In fact, billionaires have started building sprawling underground mansions in the event of societal collapse or World War 3 breaking out.
And security is seemingly a huge part of the design process, too - protecting owners from the outside world coming in.
You only have to check out a recent showcase of a doomsday bunker in Kansas on Fox Business to see how serious this is - they're not even allowed to name the facility or say where it's located, since its clients are so security-minded.
The juiciest recent details have come from an extensive piece in The Hollywood Reporter, though, which managed to talk to Al Corbi, president and founder of a firm that makes bunkers and underground systems - SAFE (Strategically Armored & Fortified Environments) - particularly about the defence mechanisms in some of these buildings.
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In one case, he said: "We wound up literally building a 30-foot-deep lake [around the compound] skimmed with a lighter-than-water flammable liquid that can transform into a ring of fire. The only access to the island is a swing bridge."
That giant ring of fire is pretty wild to imagine, but it doesn't stop there - Corbi also talked about "water cannons that can take down parachuters, Apache helicopters, whatever’s coming your way 500 feet in the air" that was designed for one client, along with digging a "tunnel through with flame-throwers, gassing systems, a steel wall that closes midway that could stop a 16-wheeler going 80 miles an hour".
Another designer described rotating fireplaces, secret entrances and escape tunnels, further cementing the fear of infiltration.
Many of these additions sound like complete death traps, something out of a JG Ballard sci-fi novel rather than the real world, but it's clear that fear of invasion and outsiders is driving much of these purchasing decisions.
The reaction has been stark, too, with one discussion thread on Reddit seeing loads of people making dark jokes about this level of planning.
The most popular comment in the whole thread was a grim joke: "That’ll make a great fortress for the rich guy’s staff after they kill him on day one of the apocalypse."
Another wondered aloud: "I find this hilarious what will these guys do in the apocalypse when tech is dead?" In fact, this is addressed in the piece, which mentions that some buyers are hoarding older tech that will still work in the event of an EMP (electromagnetic pulse) or the loss of electrical power.
So, these people really do seem to be thinking of everything - even luxurious ameneties like an underground swimming pool and a self-sufficient farm are included in one bunker.