BY SALLY MCGRANE

New York Times

One Saturday morning in Helsinki, Lassi Kurkijärvi and his 4-year-old daughter stepped into an elevator near the Hakaniemi market square. After a 60-foot descent through the city's granite bedrock, the girl took off down a tunnel to the cavernous "Leikkiluola" ("Play Cave") and disappeared into a palm tree-themed adventure maze.

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