Vallisaari

This guide to Vallisaari, an island near Helsinki, opens at the Valley of Death, the hollow where a 1937 ammunition blast killed thirteen people, including the Defence Ministry's own head of armaments. It traces the island's long life as a guarded fortress, first fortified by Sweden and later rebuilt by Russian engineers, and the sabotage theory investigators never quite ruled out. It follows the decades the island stayed a closed military zone after the blast, before opening to visitors in 2016. Keep to the marked path through the hollow; unexploded ordnance is still believed to lie in the ground.