View of Helsinki, Finland, May 6, 2017. Underneath Helsinki lies a series of subterranean passageways where Finnish soldiers and civilians alike prepare for disasters. REUTERS/Ints Kalnins

An underground city in Helsinki, Finland has further secured itself in preparation for Russia's biggest military exercise in years, reported the Wall Street Journal Thursday.

The subterranean network, already in place, prepared a line of defense for Finland's capital. Helsinki's underground city developed with many innovations such as a parking caverns and a data center. In the past few years, it adapted to better fit defense strategies. New features to fortify the city were added, such as blast door seal entrances and passageways for the military. Finnish soldiers regularly train under Helsinki. If the city was attacked, the subterranean layer — which features over 124 miles of tunnels — has enough space for Helsinki's more than 600,000 residents.

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Finland's extra preparations occurred because of the upcoming Zapad 2017 exercises in September, a military venture during which Russia will "practice detecting, jamming and targeting allied forces with drones and advanced artillery, while spreading disinformation about what its forces are doing." Russia and Belarus jointly planned the exercise. Belarusian First Deputy Defense Minister, Chief of the General Statt and Major-General Oleg Belokonev held a conference Friday, reported Belarus News Friday. At the conference, Belokonev said preparations for the exercise are entering the final phase, according to the Belarusian Defense Ministry website.

The exercise is planned to take place on Finland's border, causing worry that Russia will attempt militant action.

"More than looking at what will happen during the exercise, we're more interested in what will happen afterward and make sure that the troops actually do leave," said Jarno Limnell, a Finnish expert on cyber security and military science.

NATO officials said they will be vigilant during the Zapad military exercises, as they are the first ones Russia and Belarus will be conducting since Russia-NATO relations weakened after the Ukraine crisis.

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Preventative measures the Finnish conducted underground were a joint measure between military members and civilians. So far, the exercises allowed for the population to prepare for multiple types of disasters.

"The tunnel system we've built comes from our own lessons learned from the Second World War," said Janne Kuusela, the Policy Director of the Defense Ministry. "You need to do what you can to keep your vital functions going even if you're being heavily bombarded."