"They appear in the dense forest, in the places you can't get on the car or truck to bring any device to drill the ground. There is no any soil that should be taken from such deep holes is found.
On this pictures people go down to one of such holes but it just finishes with nothing. There are no any reasonable ideas on how these holes appear and what they are being used for."
Also here are some photos where it looks like a truck got caught in one of these holes though its not in the forest & looks like the regular road.
Source: 1, 2
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This is happening in some Russian forests, people are finding these deep holes in dense forests & places where its not possible for trucks or any other instrument or device that could be used to drill them.