Does anyone know what makes a hexagon shaped hole

[Kenjmor2006]

A few years back I was in North Carolina and walking on the side of a small mountain and found a huge rock about the size of a small house,in the middle of this rock was a hexagon shaped hole about 10 inches in diameter,I have often wondered what it was for and what made it,sorry I never took pictures as I had no camera with me when I was there.the hole was in the middle of the rock and was straight down vertically....Kenjmor2006

 

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[Treasure finder]

When I was a machinist we would drill a hole (round of course) and use a 40 ton press to push a hexagon shaped
rod into the hole. That's how they make socket wrenches then drill out the excess metal. In rock I am still scratching
my head over that one.
Rich

 

[dsty]

Hello Ken, a photo is necessary with depth, / flat bottom / sides / photo of the top looking stright down, is it close to a river where a ship might have need to tie up.