I cant post a picture but I found a flat rock in a river that has 3 holes drilled all the way through. 2 of the holes are on the flat side and are drilled through to the other side, the 3rd hole is drilled all the way through the narrow section of the rock to the other end. There is visable "pecking" in a curved shape from one hole to the other.
I know that it is definately manmade but I have never seen anything like it.