Down a steep tunnel, underneath a main square in Stockholm, Sweden, are the vast subterranean chambers of the Klara nuclear bunker.

One of thousands of shelters across the country, it was designed and built during the Cold War to house terrified inhabitants fleeing incoming missiles.

The Klara bunker is 50 metres deep and can shelter 8,000 people. While parts of it are now used as a car park, Sweden is one of few countries with functioning nuclear shelters.

The bomb, by numbers

The atom bombs dropped in 1945 on Hiroshima and Nagasaki are the only two to have been used in war.

The first released 15 kilotons of nuclear energy. The second, 21 kilotons. Between them they killed at least 129,000 people when they fell, and up to 225,000 people in the ensuing weeks.

Today's hydrogen bombs are 80 per cent more powerful.

From a high of about 70,000 nuclear warheads globally in the mid-1980s, there are now about 14,000.

More than 90 per cent are in the hands of the USA and Russia, but the UK, France, China, Israel, India, Pakistan and North Korea also have nuclear capability.

Of those latter countries, the last four have not signed up to the Nuclear Non-Proliferation Treaty, which entered into force in 1970. North Korea did, but withdrew in 2003.

Two minutes to midnight

The first atomic bomb was developed under the auspices of The Manhattan Project.

After World War II, scientists from the project founded a publication, the Bulletin of Atomic Scientists, best known for creating the Doomsday Clock in 1947.

The distance of the clock's hands to midnight is a metaphor for "how close we are to destroying our world with dangerous technologies of our own making".

In the early 1960s, after the Cuban Missile Crisis when war was only narrowly averted, the hands stood at nine minutes to midnight.

On January 25, 2018, the Bulletin placed them at two minutes off the hour, citing some leaders' "provocative actions" which they said "increased the possibility of nuclear war".

But how ready is the the world for a renewed arms race, an escalation in proxy wars and — the worst-case scenario — a nuclear war?

Cold War 'back with a vengeance'

Stairs leading down into Klara Shelter, central Stockholm. ( ABC RN: Fiona Gruber )

Sweden, neutral since 1814, has a history of civil protection and currently has 65,000 nuclear bunkers, enough for about 70 per cent of its population.

In December 2017, after a hiatus of 15 years, Sweden's Defence Commission recommended building new shelters.

Sweden has also reintroduced partial military service.

Rolf Nordengren works at the Swedish Civil Contingencies Agency (MSB) which oversees the country's civil protection and disaster relief. He says the mix of military and civil defence is part of the country's commitment to protecting its population. According to Nordengren, it's all about being ready for a breakdown of normal society, whether, through war, civil unrest, terrorism or a natural disaster.

As a neutral country that isn't part of NATO [North Atlantic Treaty Organisation], Sweden has to be self-sufficient.

Swedes, he says, would argue that neutrality does not equate with pacifism or passivity. It's about keeping your population safe from bombs and fallout, to rise out of the ground and fight another day.

"In Sweden we have total defence."

Sweden's forward planning and increased alertness hasn't come out of a blue sky; since Russia's 2014 invasion of Crimea and incursions into the Ukraine, and incursions in to Swedish air and submarine space, the country has been extra wary of its traditional enemy.

Russia's relationship with the USA and its Western allies has gone from bad to worse in the past two years, escalating over the past few months. Its involvement in the civil war in Syria, cyber warfare, alleged meddling in the elections of foreign powers, and the alleged poisoning of former Russian spy Sergei Skripal and his daughter in the UK have all contributed to the hostile atmosphere.

In April, the UN Director General Antonio Guterras warned that 'the Cold War is back with a vengeance'.

Guterras added a rider to his statement.

"The mechanisms and the safeguards to manage the risks of escalation that existed in the past no longer seem to be present," he said.

Are nuclear bunkers back in style?

Japanese companies are marketing home nuclear shelters to the general public. ( Reuters: Kim Kyung-Hoon )

Professor David Lowe, a historian and Cold War expert at Deakin University, says the variation between countries' readiness for nuclear war is huge.

Alongside Sweden, Switzerland stands out as having the best civil protection scheme, with coverage for its entire population.

"It dwarfs what other countries did," Professor Lowe says.

"Even today, if you build a new house you either have to have some form of bunker or you have to make a donation to a communal bunker."

Other countries that maintain nuclear bunkers include South Korea, but then it is, technically, still at war with its volatile Communist northern neighbour, whose nuclear capability is another very real cause for alarm.

Many countries in war-torn regions have shelters for conventional warfare and disaster, but, given the Cold War ended more than a quarter of a century ago, it's not surprising that many countries that had nuclear shelters gave them up decades ago.

In some cities you can see the old signs for them up on walls; relics of a time when nuclear war seemed very real.

In New York these old signs recently made news, when they were finally taken down in December 2017; they were deemed misleading as they pointed to shelters that had long ago ceased to exist.

Other countries, the UK included, only ever had nuclear shelters for its military, government and Royal Family.

Unless you were near an underground station — which would protect from the blast but not contaminated air — the population were given instructions on how to build a shelter by taking the doors off their hinges and making a fallout cubby house.

Inside a bunker in the United Kingdom. It's no luxury, but no radiation in your bunk bed. ( ABC RN: Fiona Gruber )

In Australia, says Lowe, despite Prime Minister Robert Menzies being keen for Britain to test and store weapons on Australian soil in the 1950s — the latter never eventuated — the country was seen as being too far away and too large for there to be a need for shelters.

In fact, even the government in Canberra lacked, and still lacks, a nuclear shelter.

"On the Beach probably sums up how Australia felt," he says, referencing the Neville Shute 's 1957 post-apocalyptic novel and subsequent Hollywood film.

The book delves into the last remnant of the world's population surviving in Melbourne before they too succumb to radiation poisoning.