The beautiful town of Yumoto Onsen is, as its name suggests, a hot spring village through and through. Nearby lakes and waterfalls are named after hot water and hot springs. Gorgeous hot spring baths (onsen) can be found in nearly all of the hotels and ryokan in the area (which is near Nikko in Tochigi Prefecture), as well as a few public ones. There is even a temple and shrine devoted to the hot springs in the town. Being such a central part of the village's tourism industry, where does all that soothing, healthy water come from?
Head to the town's northeast corner to find out. There, you can find the Yunodaira Marsh: a flat, open area that is the fountainhead of the natural hot springs that run beneath the earth. The marsh is a wet, boggy area that you can cross on wooden boardwalks. Exploring the area is quite interesting. In one corner, you can see flat wooden structures housing the pumps that send the hot water throughout the town. You can also see puddles of water all around, but unlike most puddles, this water comes upward, not downward. Many of the puddles appear to be boiling, with bubbles coming up from the ground and steam lifting off the top. That is indeed the case, although the puddles were safe to test out with your finger without getting scalded. Due to the high sulfur content of the hot springs in this area, many of the puddles had a yellow or green tinge to them, and the sulfurous smell was quite strong.
A nearby sign explains that this source of natural hot springs was discovered by the famous Priest Shodo-Shonin in the eights century. Priest Shodo is a legendary figure who is known for founding nearby Nikko, and establishing many of the famous shrines and temples there that have gone on to become UNESCO World Heritage sites.
Following the wooden boardwalk to the far corner will lead you to Onsen-ji, the Hot Spring Temple. This is a small but beautiful Buddhist temple that actually has its own public hot spring bath (which is closed during the winter, unfortunately). At the opposite end of the marsh (near the mountain), you can often see some of the many wild monkeys that live in this area. While they're beautiful and interesting, be careful, as there have been incidents of the monkeys being a little aggressive, especially when there's food around.
Yunodaira Marsh is an interesting place to poke around, and while it isn't enough to justify a trip to Yumoto Onsen on its own, visitors to the town should definitely set aside some time to go exploring.