Sixteen million years ago, an underwater volcanic eruption blanketed this region's seabed with ash, which compressed and chemically transformed into a soft rock called green tuff. Five million years later, magma pushed upward through those tuff layers from below. As the molten rock cooled and contracted, it fractured into tight hexagonal columns -- a process called columnar jointing, the same geology that produced the Giant's Causeway in Northern Ireland and Devil's Tower in Wyoming. The Kiyotsu River has spent the eons since carving through both rock types, exposing sheer walls of columnar andesite that rise in geometric perfection above turquoise pools. The gorge stretches 12.5 kilometers through the heart of Joshin'etsu Kogen National Park, on the border between Yuzawa and Tokamachi in Niigata Prefecture. In 1941, Japan's Agency for Cultural Affairs recognized these formations by designating Kiyotsu Gorge as both a Place of Scenic Beauty and a Natural Monument.

The Japanese love their ranked lists, and Kiyotsu has earned a place on one of the most coveted: the Three Great Canyons of Japan, alongside Kurobe Gorge in Toyama Prefecture and Osugidani Gorge in Mie Prefecture. Each canyon has its own character. Kurobe is alpine and remote, accessible only by a scenic railway clinging to cliffs above a deep V-shaped valley. Osugidani is subtropical and lush, sheltering ancient cedar trees in the Kii Peninsula's humid mountains. Kiyotsu falls somewhere between -- temperate, volcanic, defined by the angular precision of its columnar rock faces. In autumn, the contrast sharpens: deep red and amber maples frame those grey-green hexagonal columns, and the Kiyotsu River runs cold and clear below. The onsen resort at the gorge's entrance, Kiyotsukyo Onsen, fills with visitors who come for the foliage and stay for the hot mineral water.

A footpath once ran along the river at the base of the canyon walls, but a rockfall in 1988 closed it permanently. The cliffs are beautiful but unstable -- columnar joints fracture cleanly, and whole sections can detach without warning. To preserve access, a pedestrian tunnel was carved 750 meters into the canyon wall, with viewing stations cut through the rock at intervals to frame the gorge below. For years, the tunnel was a simple concrete passage with dim lighting. Then, for the 2018 Echigo-Tsumari Art Triennale, Beijing-based MAD Architects transformed it into the Tunnel of Light. Each viewing station became an art installation themed around one of the five classical elements. The Periscope station features a cedar-roofed cafe with a hot spring foot spa. The Drop station lines the tunnel with convex mirrors backed by fiery orange light. And at the tunnel's end, the Light Cave wraps the final chamber in semi-polished stainless steel, with a shallow pool on the floor reflecting the gorge's colors in an endless loop.

This is snow country. Niigata's mountain interior receives some of the heaviest snowfall in Japan, and Kiyotsu Gorge can be buried under meters of powder from December through April. The gorge takes on a different severity in winter -- the columnar walls streaked white, the river running dark beneath ice shelves, the tunnel entrance framed by snowbanks taller than a person. Nobel laureate Yasunari Kawabata set his novel Snow Country in this region, capturing the isolation and stark beauty of the mountains around Echigo-Yuzawa. The Joetsu Shinkansen now delivers visitors from Tokyo in about ninety minutes, but step off the train and into the gorge, and the modern world falls away. The columns of cooled magma hold the same geometry they have held for five million years, indifferent to the seasons that paint and repaint them.