Rather than being utilitarian, the spaces which are buried deep underground in an abandoned missile launch base in Kansas in the US, include a swimming pool and a cinema.

“We took a weapon of mass destruction and turned it into the complete opposite,” Mr Hall told the BBC.

Mr Hall said 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.

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,” said 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.

“We took a weapon of mass destruction and turned it into the complete opposite,” he added.

“Everyone comments on how well they sleep here.”