Media playback is unsupported on your device Media caption "We took a weapon of mass destruction and turned it into the complete opposite," says Larry Hall

Larry Hall leads the way out of the elevator and into an apartment, one of several in a new development that he has recently built.

The space is elegantly furnished. Mr Hall says the standard of the finishes and attention to detail has met with an enthusiastic response from his customers.

"When they come here for their walkthroughs and the closings, I've had several clients that literally cried when they saw it - with joy."

But there's something very unusual about these dwellings. They are situated many metres below ground in an obsolete nuclear missile site, in the middle of Kansas in the US.

Mr Hall calls his development Survival Condos. "These are luxury, nuclear-hardened bunkers that are engineered… to accommodate not just your physical protection but your mental wellbeing as well," he says.

Image caption Larry Hall has already completed one complex and has just started work on another

Despite the fact that the apartments cost millions of dollars each to buy, starting at $1.5m (£1.2m), Mr Hall says he is seeing strong demand from wealthy customers.

It seems extraordinary, not least because nearby above-ground homes (admittedly without protection against nuclear bombs) can be had for a tiny fraction of the cost of one of these bunker units. So how has he managed to build an apparently successful business selling "survival chic"?

It all began after the 9/11 attacks in New York in 2001. At the time Mr Hall was an entrepreneur with an internet business; he also had experience of designing and constructing computer data centres.

Demand for computer back-up facilities grew after the terrible events of 9/11, says Mr Hall. This gave him an idea: make a data centre that could withstand a nuclear bomb attack.

Image caption "It's like a miniature cruise ship," is how Larry Hall describes the interiors of the nuclear bunker

Potential clients showed interest in such a facility. It was a short step from this to the notion of offering bunkers that could give long-term protection for residents against nuclear war or other disasters.

The Kansas site eventually chosen for the venture used to house an intercontinental ballistic missile.

There are many such abandoned missile launch bases across the mid-West, but Mr Hall says only a small number were in a fit state to be realistic prospects for the project. Even then, he says, the construction and engineering challenges involved in building the facility were daunting.

Still, the site had one huge advantage - it came with built-in protection against nuclear bomb blasts. If you had to build this element from scratch today, "you'd better have a very thick cheque book, it's very expensive," Mr Hall says.

Image caption The bunkers include window-like high definition screens displaying views of what is happening above ground

Mr Hall says he has spent millions on providing the complex with every possible feature to keep residents safe both now and for an indefinite period, should a catastrophic event occur.

These include air and water filtration systems, a range of energy sources (including wind power), and the capacity to grow plants and breed fish for food supplies. Armed guards patrol the entrance.

Image caption They also include access to a communal swimming pool

There are many other features too, such as a cinema, swimming pool, surgery, golf range, and even a rock climbing wall. "It's like a miniature cruise ship," says Mr Hall.

He believes that luxury touches like these could help to explain a development that may seem a little surprising.

At first, he says, clients saw owning an apartment as "like life insurance", just something to be used in case of an emergency. But now some purchasers have come to regard their apartments as second homes, making regular use of them for weekends or longer breaks.

"Everyone comments on how well they sleep here," he adds.

Image caption The outside and inside of the nuclear bunkers are constantly monitored

Mr Hall is by no means the only player in the specialist market for survival bunkers, with rivals offering facilities at several locations across the globe. But with his use of luxury elements, experts say, Mr Hall is exploiting a growing trend.

"There's a market now because the traditional idea that somehow… you should 'rough it for the sake of your soul' is disappearing," says Peter York, an adviser to many large luxury businesses.

In this market, he says, "you're hitting a cohort of rich people who don't value the idea of even temporary Spartan-ness - they want everything to be luxurious all the time."

Even though he may be pushing at an open door, Mr Hall says he faces many challenges in running his business, over and above the obvious difficulties of building the facilities in the first place.

One of the biggest issues is marketing. His clients, many of whom are wealthy, tend to be secretive.

"Saying that you own a bunker apparently, for whatever reason, is just like saying you saw a UFO. A lot of these people have learned that they don't want other people to know that they have a bunker," he says.

Then there is the question of how residents would get along together in the aftermath of a catastrophic event.

Image caption "It's a lot of fun building these," says Larry Hall

Mr Hall says he has put a lot of effort into researching the psychological aspects, with the aim of ensuring that the underground community would function well in times of calamity.

Measures taken include special lighting, and varying ceiling heights, with high ceilings in communal areas, and lower ones in the medical wing "where people may feel vulnerable", says Mr Hall.

In a long-term situation where the complex was secured against the outside world, residents would rotate various jobs on a monthly basis.

"One month you might be working in the general store, the next month you'd be working in the hydroponics, tending the plants," says Mr Hall. This would help to keep people occupied, as well as ensuring that, over time, "they have a complete understanding of the facility".

Despite the challenges involved, Mr Hall appears to enjoy himself, with one complex already completed, and another one well under way. "It's a lot of fun building these," he says.