Inside Finland's underground city that can shelter nearly a million people if invaded by Russia

Helsinki's Hakaniemi Market Square has an underground indoor playground called Play Cave.

A blue triangle on an orange background marks Finland's bomb shelters.

Finland has built one of the world's most advanced underground civil defence networks beneath Helsinki.

The network includes nuclear-resistant bunkers and can shelter nearly one million people.

Bomb shelters are hidden beneath everyday places like swimming pools and playgrounds.