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'