Helsinki, Finland (CNN) On the busy streets of Finland's capital, Helsinki, security preparations are underway to host a much-anticipated summit between US President Donald Trump and Russian leader Vladimir Putin.

But deep beneath the streets, hewn into the granite, lies a vast network of tunnels and caverns that provide this Nordic nation with its last line of civilian defense.

Helsinki's network of tunnels can fit the city's entire population.

In the event of a military attack, officials in Helsinki say the entire population of 640,000 could be sheltered in the solid rock bunkers, equipped with food, bedding, sanitation, hospitals -- and even an underground ice hockey rink.

"We are taking care of the whole population of this country," said Karim Peltonen of the Finnish National Rescue Association, which manages the tunnels.

"When we are defending the country we also have to have the appropriate civil defense capability."

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