(HELSINKI) — Finland may be world’s happiest country — at least on the surface. But Helsinki’s “underground city” tells a different story.

Beneath the capital, a massive network of bunkers and tunnels spreads out all across the city. There are than 5,000 bomb shelters in Helsinki — enough to shelter more than the city’s entire population — and more than 50,000 bunkers across the country, according to Helsinki’s Civil Defense Department. All buildings above a certain size are required by law to have their own bunkers.

“There’s a historic sense that you should always be prepared. It might not be this generation or the next generation, but Russia is likely to attack Finland in some way,” said Charly Salonius-Pasternak, a leading researcher at the Finnish Institute for International Affairs.

With so much ground going unutilized, the city of Helsinki has converted some of its shelters into spaces for everyday public use. There’s an underground playground, a shelter that doubles as a hockey rink, even an underground swimming pool.

This is what it’s like being Russia’s neighbor. The two countries share an 800-mile border and a long, complicated history.

For decades, Finland opted not to join any military alliance in an effort meant to appease Russia’s security concerns. As a result, Finland had to ensure it could fend for itself. So it’s not just the bunkers; conscription is still mandatory for men, and the country has about 900,000 reservists.

“We have to take care of the citizens, that’s the main reason we have this system,” said Tomi Rask, an instructor with Helsinki’s Civil Defense Department.

But the very scenarios Finland has spent years preparing for are now playing out in Ukraine, where some have been living underground for weeks.

The invasion marked a turning point for Finland-Russia relations. Public support in Finland for joining NATO is skyrocketing from roughly 30% before the war to more than 70% in the weeks after the invasion.

“We have such a horrible neighbor on the east side of Finland. We don’t have any other option than to go to NATO,” said said Finland citizen Kare Vartiainen, who ABC News met making use of the underground pool.

After years of neutrality, on Thursday, Finland’s leaders announced Finland should apply to join NATO “without delay.” Sweden is expected to follow suit.

The country’s accession would more than double Russia’s land border with NATO. It would also expand NATO’s influence in the Arctic and further unify the West, said Salonius-Pasternak. NATO would also grow stronger.

“NATO would now have two more old democratic countries, both with really capable militaries, so that effectively all of northern Europe would now be one region to defend,” he said.

There are those that are still skeptical, like Veronika Honkasalo, one of the few members of Parliament who doesn’t think Finland should join. MPs are expected to take up the issue next week.

“I’m afraid that NATO membership will increase actually the tensions in the Baltic Sea region and also will increase the tensions in Finland, especially regarding the eastern border,” she said.

Russia has already threatened “serious military and political consequences” if Finland and Sweden join NATO, saying it will have to bolster its defenses in the region and that it could decide to place nuclear weapons in the Baltics.

There are concerns about what could happen in the time period after Finland and Sweden submit their applications but before they formally join the alliance. The two countries now hoping to win over security assurances from allies, including the U.S.

On Thursday, U.K. Prime Minister Boris Johnson pledged to come to Finland and Sweden’s aid if either nation is attacked.

Finns say now is the time to act while Putin is busy with Ukraine. The war in Ukraine is prompting Finland, even with its 50,000 shelters and capable military, to decide it can no longer go it alone. It’s likely giving Putin the very thing he worked so hard to prevent: NATO’s expansion.

“We are a small nation, we need help, we need friends. And from my point of view, maybe NATO is the friend that we need,” said Rask.

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