Tiangou Geyser (中文版: 天狗溪噴泉) is a wild hot spring in Yilan, Datong Township. Super-heated steam erupts from the ground sending columns of water into the air as high as 8-10 meters. After the eruption the water drains completely and the cycle starts again. This happens approximately every 10-15 minutes. Warm water also seeps out of the rocks and riverbed in many locations, but it's only around 35C, which is not hot enough for for a proper hot spring bath.
Getting to the geyser is a short 2km hike up the river. The hike is relatively flat and easy, but crosses the river multiple times, so it could prove difficult when water levels are high.
Across the valley from the geyser there is also a small waterfall. It's a 250 meter hike up to the waterfall, which takes about 5 minutes.
Yilan County, Datong Township