Arizona is still one of the states that has places almost unknown or totally unknown by the general public. Of course, you still will need to drive long dirt road.
Coal Mine Canoyn is the one of such places. You can find it in many sources but still I didn’t see the photos of it on popular sites.
The canyon is on the boarder between Hopi and Navajo Indian Reservations. And because of that there is no need of a permit. But I read somewhere if you want to go down to the canyon, you need to contact Hopi.
There is no formal trail. We hiked along the canyon and it was totally enough for us.
There is no need to do a lot of sofisticated hiking to find a lot of amazingly colorful formations. The colors, forms and textures of the eroded sandstone are something special. It could remind one of Zion or Bryce but on the less scale and non crowded.

The access road to canyon starts 16 miles from Tuba City from hwy 264. Google Maps know about the canyon but the navigation tells to turn earlier. The private land borders the canyon, and tehre are some dwellings right near the parking lot. When we visited the cows grazed near the canyon. But we didn’t see anyone near them.