The sky was a brilliant blue and the crisp air of early autumn filled the lungs of those stepping out of their vehicles that September morning. Conditions were perfect for mountain climbing.

Inside the ropeway cars there was cheerful chitchat, some hikers carefully adjusted their kit while others stared out of the windows as Nagano Prefecture slowly spread out like a sea of green below them to the east.

Stepping out into the fresh air high up the mountain at 2,150 meters, there was only the sound of the wind and birdsong, the faint creaking of the ropeway soon left behind as hikers followed the trail that snakes through the trees up toward the barren summit plateau of Mount Ontake. Then, just before midday, the top of the mountain exploded with a searing fury, act one of what would become Japan's deadliest postwar volcanic disaster.