Huoyan Mountain sits at the border of Sanyi and Yuanli townships in southern Miaoli County, rising on the north bank of the Da'an River. Geologically, it belongs to the Toukeshan Formation, a Pleistocene-era geological unit laid down between roughly 2.6 million and 11,700 years ago. What makes Huoyan distinctive is the combination of forces acting on it: tectonic uplift has pushed iron-rich laterite soils toward the surface, while the underlying rock has poor shear strength — it fails easily under stress. Add Taiwan's heavy monsoon and typhoon rainfall, and the result is a mountain that erodes dramatically and continuously. Vegetation cannot establish itself fast enough on the steep, unstable slopes. The red rock stays exposed, baking in the sun, giving the mountain its fire-colored appearance year-round. The summit reaches approximately 602 meters.

Huoyan stands at a geographic and administrative boundary. To the north, Miaoli County; to the south, Taichung. The mountain marks the edge of the Sanyi area and anchors the northeastern boundary visible to travelers on the old and new mountain railway lines that thread this terrain. From the train window, the red cliffs emerge suddenly as a visual shock — a bare, jagged ridge above the tree line, with gullied ravines carved into its flanks. The effect is especially dramatic in afternoon light, when the western-facing slopes glow against the darker greens of the surrounding forest. County Highway 140, which links the coastal lowlands with the mountainous interior of Miaoli, passes along the mountain's south side. The highway was historically vulnerable to landslides triggered by typhoon rains; in 2006, a tunnel was constructed to protect the road from repeated blockages — infrastructure bending around the mountain's instability rather than trying to stabilize the mountain itself.

Taiwan's Forestry and Nature Conservation Agency has placed Huoyan Mountain under protection, recognizing the landscape as both ecologically unusual and visually irreplaceable. The bare laterite slopes support a specific ecology adapted to hot, dry, nutrient-poor conditions — hardy shrubs, grasses, and pioneer species that most mountains in Taiwan's humid climate never develop. Conservation here means accepting the erosion rather than fighting it, preserving the dramatic bareness as part of what the mountain is. At the same time, Huoyan has been embraced as a recreational destination. It appears on Taiwan's list of the 'hundred little mountains' (臺灣小百岳), a Sports Administration initiative promoting accessible, rewarding hikes at shorter summits. The designation has made Huoyan a standard entry on Taiwanese hikers' checklists.

Two hiking trails climb to the triangulation point at the main summit. The routes are not long — Huoyan is a little mountain, promoted as such — but they are steep, and the exposed laterite underfoot can be slippery when wet. Trekking poles earn their keep here. The trail experience combines dense subtropical forest on the lower slopes with the sudden openness of the bare upper ridges, where the red soil stretches away and the views extend across the Da'an River valley toward the coast. The north peak of Huoyan stands slightly higher than the main summit but is not accessible by the maintained trails. It remains as the mountain reserves something for itself — visible, but not reachable by the usual routes. The Huoyan Mountain Ecology Museum near the base offers context for what visitors see on the slopes: the geology, the ecology, the slow drama of a mountain that refuses to stand still.