Numajiri Onsen is a hidden gem deep in the woods of the Adatara Mountain range, accessible only by a 45-minute hike through a forest trail.

Trailhead: https://maps.app.goo.gl/ewxcUvksoswk4chSA

This natural onsen is one of my favorite spots because it offers a truly one-of-a-kind experience. I've visited three times, and each visit is slightly different, as the pools and temperatures vary with the seasons. The pools themselves also differ—some are shallow, making them ideal for resting your head on the rocks, while others are deep enough to fully submerge and enjoy a full-body soak.

The water is rich in minerals, especially sulfur, so expect to carry the distinct onsen scent with you afterward! It's a wild, open-air onsen, surrounded by a stunning volcanic landscape with steam vents nearby, making the experience feel incredibly immersive. One of the best parts is that it’s free, but be sure to bring a swimsuit as hikers frequently pass by.

The onsen is open from mid-April to mid-November, so it’s best to visit during this period for optimal conditions.