Sankukai is a truly a 秘密湯 secret bath. It's a 30 minute walk from the nearest bus stop far, far up in the mountains north of Osaka. The water is amazing and hot. Be careful washing off or touching the hot faucets, it could burn. There are two simple concrete tubs a hot and warm one. There's also a small metal cold water tub in the corner. The cold water is also from a well. This is a real onsen experience and well worth the trip to enjoy it.

Update Fall 2019: I walked here from Myokenguchi. Took a good hour or so. Great walk and well worth it. The bridge and some of the buildings were washed out during a storm. Thankfully repaired. Amazing onsen but please be respectful of the place and the people who care for this place.