500+ shelters 25m below capital can hold 900,000, more than entire city - with cafes and go kart tracks

Their doors are so thick a nuclear blast wouldn't leave a dent, officials say of bunkers built during Cold War

City dug 9million cubic metres of bedrock - 50,000 double-decker buses, for shelters with weeks of supplies

Helsinki's 25 metro stations can also be transformed into urgent escape routes

Emergency bunkers are receiving new attention as Finland's NATO bid meets Moscow tough talk

Kremlin has vowed 'retaliatory steps' including moving nukes closer to Europe

But officials in Helsinki's emergency unit say Finns are unfazed by Putin threats.

Official tells MailOnline: 'As a neutral country, we have always felt the need to protect ourselves - and we do'