The spectre of nuclear war is looming, with Vladimir Putin broadening the criteria under which Moscow can deploy its fearsome atomic arsenal and Kremlin mouthpieces threatening to sink the UK under a 'radioactive tidal wave'. Sweden has warned its citizens to seek shelter in the event of a nuclear attack, while Norway and Denmark have issued advice on taking iodine tablets to protect against radiation in the event of a nuclear accident. Among the mega-rich, more drastic preparations are being made, with many more investing in life-saving but also luxurious bolt-holes to see out doomsday over recent years.

'It got really busy, and it seems like the phone hasn't stopped ringing; World War III seems like it's coming,' Ron Hubbard, CEO of Texas-based Atlas Survival Shelters said earlier this year. He was responding to news that Facebook boss Mark Zuckerberg was building a 5,000-square-foot bunker under his Hawaiian ranch, something the shelter entrepreneur said that prompted 'a buying frenzy.' Appetite for secure underground lock-ups also increased amid uncertainty around the US election, bunker brokers claim, with sales reportedly booming over the past few months.

Hubbard's is among a host of firms which stand ready to meet the increased demand, as the global security situation becomes increasingly uncertain. Others include Rising S, which says it has built 14 bunkers in New Zealand; Survival Condo, which converts abandoned US missile silos into underground apartment blocks; and.