What makes Thermal Valley scientifically remarkable — and historically prized — is the unusual chemistry of its water. The spring is classed as a green sulfur hot spring, one of only two such formations globally known to contain radium. The mineral cocktail includes alunite, jarosite, realgar, and native sulfur, all byproducts of volcanic hydrothermal activity beneath the Tatun Volcanic Group that underpins this corner of Taipei. About 150 meters downstream, the acidic water once reacted with andesite riverbed rock to produce layered, cream-colored crystals shaped like diamonds. Scientists named these crystals hokutolite — Beitou stone — the only mineral in the world named after a region of Taiwan. The radium content that helped create them is the same element that earned Thermal Valley its place in Japanese colonial-era guidebooks as a site of scientific curiosity as much as scenic wonder.

During the Japanese colonial period, Taiwan's governor-general administration promoted a list of Eight Scenic Views and Twelve Attractions across the island. Thermal Valley made both lists — celebrated under the lyrical name "Jade Mist of the Sulfur Spring." The juxtaposition captures the place perfectly: the official designation was poetic, the local nickname blunt. Beitou itself had been developed as a hot-spring resort town since the 1890s, when German merchant Ouland first noticed the springs' potential. Under Japanese administration the area gained bathhouses, inns, and a spa culture that persists today. The Thermal Valley pool, too dangerous to bathe in, became instead a viewing attraction — a spectacle at the neighborhood's upper edge, where volcanic heat expressed itself most dramatically.

The Tatun Volcanic Group, which also forms the backbone of Yangmingshan National Park directly above Beitou, is the geologic engine behind everything visitors see and smell here. The group comprises more than twenty volcanic peaks, most dormant but not extinct — recent research has identified a magma body beneath Seven Star Mountain, reclassifying Yangmingshan from dormant to potentially active. Hydrothermal water percolating through fractured andesite picks up sulfur dioxide and hydrogen sulfide from those deep volcanic sources, emerging at Thermal Valley already acidic and mineral-rich. The spring's vivid green color comes from the dissolved sulfur compounds and fine particulates refracting light in shallow water. On cooler mornings, when the temperature differential between pool and air is greatest, the steam rises in dense white columns visible from the road below.

What is most surprising about Thermal Valley is its accessibility. The Xinbeitou branch of the Taipei Metro deposits visitors within easy walking distance; the route passes the Beitou Hot Spring Museum — itself housed in a colonial-era public bathhouse — before climbing gently up Hot Spring Road toward the valley. The park around the pool is compact: a paved path traces the perimeter, interpretive signs explain the geology, and a small pavilion offers shade. There is no bathing, no soaking, no commercial development pressed against the water's edge. The restraint is appropriate. This is a place to stand at and regard — to take in the color, the heat shimmer, the sulfur air, and the reminder that beneath Taipei's modern skyline, something volcanic is still at work.