Inside the Nordic nation that's spent decades preparing for another Russian invasion.
They're blasted into the bedrock, deep below the streets of Helsinki.
Finland's capital has a population of around 680,000, but there's enough space for 900,000 in this underground labyrinth of 5,500 bunkers and tunnels.
Right now, people can play sports and relax in a cafe down here while their children run amok in the playground. Some have swimming pools and one even includes a go-kart track.
Their primary function, however, has nothing to do with leisure.
"This is the backbone of our safety system," said Tomi Rask, a local official who's showing us around. "It is intended for war-time use, but it has a dual purpose."
Some people in Finland, which shares a large border with Russia, believe the country could need to use them for safety soon.
The Merihaka Civil Defence Shelter, which we've been given access to, is an elaborate subterranean facility which can fit up to 6,000 people in the event of an attack.
It's among the city's biggest bunkers and includes sophisticated ventilation systems, as well as plenty of medical supplies, bunk beds, toilets, and water.
In a matter of hours, the sport and recreation facilities can be packed away.
Blast-proof doors then seal it off from the outside world and even, "theoretically", ensure it withstands the force of a nuclear bomb.
"There has always been a threat of war because of our neighbours ... so these shelters are the last resort for the worst-case scenario," Mr Rask, from the Helsinki City Rescue Department, said.
Finns concerned about Russian President Vladimir Putin ordering an attack point to two things: history and geography.
Their country shares a 1,340-kilometre land border with Russia which is now three-and-a-half years into its full-scale invasion of Ukraine — another of its neighbours.
Moscow has also previously attacked Finland.
In 1939, the Red Army launched an assault which became known as the "winter war".
When a peace treaty was signed, Finland retained its independence but was forced to cede almost 10 per cent of its territory.
Finland's Foreign Minister Elina Valtonen told the ABC her country's history of conflict with Russia meant citizens remained cautious.
"We were luckily able to fend [the Soviets] off, but at the same time, what has remained is this sort of cautiousness that we are strongly aware that Russia might be a threat again at some point in the future," she said.
"You have to prepare for all sorts of crises in order to be able to survive, essentially, and not just war. There are also different kinds of conflicts.
"For a small nation, preparedness is a matter of existence. Finland has learned it the hard way."
Strong support for mandatory military service
Another pillar of Finland's defence strategy is mandatory military service.
Every male citizen aged between 18 and 60 must do a period of training — or an alternative civil service — while women can sign up voluntarily.
While most go on to pursue careers outside the armed forces, Finland now has around 900,000 reservists ready to serve, if required. That's roughly 20 per cent of the country's total population.
At a base on Santahamina island, near Helsinki, new conscripts are put through their paces year round.
Many young men see it as a rite of passage.
"We need be ready to defend ourselves and our land and what we have. We have to defend our government and the things that we believe in, because if we don't do that then there's no point in having it," said 21-year-old Vincent Holmström.
Julius Vayrynen, 20, agrees.
"I'd be willing to fight, definitely," he said.
"There's a lot that I love about this nation and there's lot that I want to protect."
While many European countries scrapped the concept after World War II, there's evidence to suggest mandatory military service has widespread support in Finland.
A recent poll asked, if the country was attacked: "should Finns take up arms to defend themselves in all situations, even if the outcome seemed uncertain?"
Seventy-eight per cent of respondents said "yes".
Before Russia's full-scale invasion of Ukraine in February 2022, Finland was a neutral country.
But that attack prompted it to lurch further to the West and join the North Atlantic Treaty Organisation (NATO).
The same year it also began building a 200-kilometre-long fence along part of its border with Russia. By the end of 2023, it had closed all land crossings indefinitely.
Previously, Finns could go to Russia to buy cheap petrol, and Russians would venture into Finland for its superior shopping.
Communities that relied on the once-bustling crossings are hurting.
The Finnish border town of Imatra, which is closer to Saint Petersburg than Helsinki, used to get masses of Russian visitors.
"The city centre is now quite quiet ... and some people have had to move away because the work is gone," Mayor Matius Hilden said.
"The effects for the tourism have been huge. Russian people used to shop here. There were big shopping malls for the Russian tourists here."
In the centre of town it's hard to miss Tiina Kärkäs-Sund's clothing and gift shop — partly because it's painted in bright colours, but also because it's one of the only businesses still open.
She's had a store in Imatra for 32 years and has no plans to shut, but she feels sorry for those businesses who have had no other choice.
"If we look out the shop window now you can't see anybody there," Ms Kärkäs-Sund said.
"I think about 20 shops have closed down. A lot of people are quite sad about this situation, and of course it is sad for us too."
Despite the downturn in Imatra, both Ms Kärkäs-Sund and Mayor Hilden said the overwhelming opinion among people in the region was that the border should remain closed.
"Russians have to take the responsibility of what they have done, so I think the majority of Finns don't want Russian tourists until there is peace in Ukraine," Ms Kärkäs-Sund said.
"I would like to see the border reopen when the Ukrainian war is over and Russians start behaving."
Russia's military expansion near border
Some analysts argue Russia's thirst for war may not stop at Ukraine.
Last month, for example, at least 19 of Moscow's military drones entered Polish airspace.
Poland's Deputy Prime Minister Radoslaw Sikorski said the incident represented an "unprecedented" attack on not only Poland "but also the territory of NATO and the EU".
Then, on September 19, three Russian fighter jets entered Estonian airspace over the Gulf of Finland for over 10 minutes.
A NATO spokesperson said it was "yet another example of reckless Russian behaviour".
Emil Kastahelmi is a military analyst with the Finland-based Black Bird Group which monitors satellite vision and open-source war footage.
He says satellite imagery provided by Planet Labs shows the Kremlin has been expanding military assets near its borders with NATO countries, despite the significant number of units and infrastructure deployed in Ukraine.
Russia's Luga base is only around 250km from south-eastern Finland and even closer to Estonia, which is also part of the alliance.
Mr Kastahelmi said there was evidence to suggest a small brigade had been turned into a larger division there, and that various vehicles and artillery had been brought in.
"In the last maybe a year or two, Luga now has actually more military equipment than it had before the Russian invasion of Ukraine," Mr Kastahelmi said.
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Meanwhile, a Russian base at Kandalaksha, just over 100km from the Finnish border in the north, was also being expanded.
"These images are eight months apart and they are showing rather extensive construction work ongoing at the moment," Mr Kastahelmi said.
"There's expansion and building new accommodation, storage areas, space for vehicles.
"There's an artillery brigade and an engineering brigade going to be stationed in this area."
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Mr Kastahelmi said all of this meant there was the potential for tens of thousands of Russian soldiers to be stationed near Finland's border in the future.
"There's no acute threat at the moment rising near our borders in a way that the Russians could actually build up a force for a conventional invasion against NATO," he said.
"However, in the long term, Russia is an aggressive dictatorship which attacks and threatens its neighbours, does continuous sabotage operations, hybrid operations, airspace violations and so on.
"So of course we need to continuously be aware of all potential scenarios.
"We simply do not know what kind of thinking there is on the Kremlin side, what their next steps are, how far are they willing to actually go, and what they are willing to risk in order to expand their influence."
Back in the bunkers under Helsinki, many people are hoping the sporting equipment and leisure facilities won't need to be moved any time soon.
"By legislation we have 72 hours to convert all the shelters to war-time use. But we have been practising in this shelter and we can convert this in somewhere from six to eight hours," Mr Rask said.
While politicians ramp up warnings about Russia's threat, not everyone is quite as worried.
Mr Rask has a simple take on his country's increasingly aggressive neighbour: "It has always been there and it will always be there."
But he also has a message for Russia.
"Finland has always been here and we will always be here too," he said.
Credits:
Reporting:
Camera operator: Daniel Pannett
Digital production:
Satellite images:
Map: Institute for the Study of War and AEI's Critical Threats Project
