OK, one vid is worth 1000 words:
This is in Kamui.
Cherry Bomb did not have any effect either.
I do know it's the entrance to the third and final devil gate.
I do have the Digging Champ dojo technique.
I was able to dig the other two devil gates just fine.
I am well aware that in the above link it says:
The Devil Gate Trials will be open after defeating Lechku & Nechku in the Wawku Shrine
Indeed, I still did not reach the shrine. However, further search gave me this walkthrough that says:
The Spider Bandit can be fought after blooming the guardian sapling. The Devil Gate will be active after clearing Wawku Shrine.
Which means I can fight the spider, I have already bloomed the sapling... but I just can't dig my way down to the gate!
Any idea how I can dig this?