By the time I reach Mount Aso’s summit, it has been raining all day, often in torrents struck through with lightning followed by near peals of thunder.

In this weather, guide Yoshifumi Usui explains, the summit is usually closed to visitors — not for the threat of eruption you might normally fear from this active volcano but for the thunderheads passing at level altitude. From the purpose-built cement bunker we’re huddled inside 1,500 kilometers above sea level, the danger isn’t due to lightning striking from above as much as from within the thunderheads astride us.

It’s a strange moment: I’d flown into Kumamoto and driven an hour into central Kyushu excited to peer down into the bubbling, smoking, sulfurous crater of one of Japan’s largest volcanoes, yet here was an entirely different force of nature holding me back.