I am not sure why, but not far from the main street leading to Myoken onsen you can find an interesting cave with painting on the walls. While they seems to have seen better days, there is stairs leading to the cave. The walk is not very long and once on top, it is hard to miss and he trail is a dead end leading to a stone wall in the mountain. There is also a torii gate if you needed anything more obvious.
Once you get to the entrance, check on the right, there is a switch to open the light inside (don’t forget to close it). You will have to crouch down to get in, but the insider is quite spacious, but it is limited to a single big room in the cave. There is painting on different parts of the walls, mostly swirly patterns painted in different colors. Searching online, I could see that the cave is where the leader of he Kumaso tribe lived, before being killed by Yamato Takeru, a legendary hero who would have lived around the first or second century. The painting was done by the artist Hagihara Sadayuki. Despite knowing that, I am n’est-il not sure why make such painting inside that specific cave, but regardless, it is a pretty interesting spot to stumble upon.
Just as a fair warning, there was a bat that got scared when I entered and it flew for a minute before finding a safe place to land. Despite the light, the cave is still a bit dark, so you might want to bring a tripod if you are looking to take pictured.
While I would not do a detour just for it, there is no reason not to go if you are stopping in the area.
A fire minutes walk from Myoken onsen bus stop.