Survivalists are buying underground doomsday bunkers to prep for the apocalypse. Here's what they look like.

The Vivos Group advises against building a shelter in your backyard and suggests instead that the location of a doomsday bunker be kept secret.

Building a bunker in a remote area also ensures that it's far from military targets. If Russia were to launch a nuclear attack on the US, Russian state TV has identified a few places that could be hit, including the Pentagon, Camp David, Jim Creek Naval Radio Station in Washington, Fort Ritchie in Maryland, and McClellan Air Force Base in California.

The Survival Condo Project operates a 15-story bunker in Kansas. The facility is located at a former missile silo, so it extends hundreds of feet below ground. It's designed to shelter the ultra-wealthy during an apocalypse, but until that moment arrives, people who've bought space there are free to come and go as they wish.

The Vivos compounds have steel blast doors — the type used in nuclear power plants — which are typically made of concrete encased in a steel envelope.

Vivos' concrete bunkers in South Dakota are designed to withstand a 20-megaton blast a few miles away. Bunkers made of fiberglass or corrugated metal may be too flimsy to withstand a nuclear attack or climate disaster.

The Survival Condo Project comes with armed security guards that patrol the grounds. In the event of a disaster, the guards can hop into SWAT team-style trucks and retrieve residents within 400 miles.

The Survival Condo Project has stored enough freeze-dried food in its bunker to last each resident five years.

The hydroponic farm uses lamps to grow produce. The company also raises tilapia in fish tanks to feed current residents.

Humans can only go about three to four days without water. So at the Vivos compound in South Dakota, water gets pumped from underground wells into stainless-steel storage tanks.

It's a good idea to have a readily available supply of basic necessities like band-aids, disinfectant wipes, and any prescription drugs you take.

Although some people keep stores of potassium iodide pills, which block the absorption of radioactive iodine in the body, those won't get you that far in a nuclear disaster — radioactive iodine only accounts for about 0.2% of your overall exposure to radionuclides after a nuclear explosion.

At Vivos bunkers, residents are asked to send any personal medications to be stored there. They're also screened for contaminants and viruses before they enter the bunkers.

Read more: 7 things the US government says you should never do after a nuclear explosion, from using conditioner to hiding in your car

To supply electricity, bunkers need an underground power generator. They also have to stockpile fuel to keep the generator running.

Rising S Company, a bunker manufacturer, offers an "NBC" air filtration system that it says is capable of filtering out nuclear, biological, and chemical contaminants.

This system is advertised as protection from nuclear fallout — the clouds of radioactive dust and ash that disperse into the atmosphere after a nuclear explosion. The company also says it the device can filter out harmful gases released by chemical weapons, like sulfur and cyanide.

Those who want to lease Vivos Group's private bunkers in South Dakota and Indiana pay $35,000 up front, then a $1,000 annual fee after that. The bunkers can fit around 10 to 24 people. Private apartments in the German complex cost $2 million to buy.

The Survival Condo Project is even more expensive — around $3 million per unit to purchase (including furniture and a three-year individual food supply). A typical unit is 1,820 square feet, so the company estimates that it can fit six to 10 people.

Interest in the Survival Condo Project "seesaws with who's in the White House," the company's founder, Larry Hall, previously told Business Insider.

"When the rhetoric was hot and heavy between Trump and Kim Jong-un in North Korea, there was a real big spike in phone calls," he said.