Fujido Cave

Ueno is the least populated municipality in Gunma, deep in the mountains on the Saitama border, and Fujido is the reason to drive there. It is the largest limestone cave in the Kanto region — 2.2 kilometres of passages, of which about 700 metres form the visitor route — and a designated prefectural natural monument. The walk through takes around 40 minutes and passes through a chamber the guides call the Great Hall, where a massive column of stone rises through the space, along with side passages narrow enough to squeeze through.

Getting to the entrance is part of it: the approach runs through a long spiral tunnel bored into the hillside, and the cave sits below the Tenku Kairo, a suspension bridge strung high across the valley. Because Ueno is a genuinely remote village, the site never feels crowded even in summer, and the cave holds a steady cool temperature all year — which locals treat as an August destination rather than a rainy-day one.

The limestone that formed Fujido built up over an immense span of geological time as marine sediment, later folded and uplifted into the mountains that now define Gunma's border with Saitama, and the same processes that carved this cave's passages are responsible for the karst topography scattered elsewhere across the prefecture in smaller, less developed pockets. Ueno's extreme depopulation — it has held the lowest population of any municipality in Gunma for years — means the cave and the suspension bridge above it function as rare economic anchors for a village that would otherwise have little reason for outsiders to pass through, and local pride in the site runs correspondingly high for a place this size. Visitors who make the drive tend to treat it as a full detour rather than a stop along some other route, since Ueno isn't really on the way to anywhere else.