Atun Valley Cave is found north of the road between Lake Floria and Lurelin Village.
Cave Location
Atun Valley Cave is located at 2453, -3193, 0066, which is on the north side of the road that goes between the Floria Bridge and Lurelin Village.
Cave Features
The first room features a pool of water and some ore deposits. There is also a wooden treasure chest in the water that you can pick up with Ultrahand. It contains a Bomb Flower.
There is a hole above the water where a Horriblin platform can be seen. Use Ascend to reach the area above, then defeat the three Horriblins to unlock their treasure chest. You can use Ascend on one of the parts of their platform that sticks out, or just place a wooden plank as a ramp up to the platform. There is also a metal treasure chest on their platform.
Near the Horriblin platform, break down the boulders in the wall to find this cave's Bubbulfrog.