He said the drill diameter would be reduced from 28 inches to 26 inches until the hammer had cleared the hard rock but added that the slightly smaller diameter would not cause problems for the rescue.

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Inside the riser pipe is a space about half a meter in diameter, large enough to hold the spinning drill pipe and bit and to allow the return flow of drilling fluid.

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The drill would use that hole as a pilot and widen the diameter to about 28 inches, which officials have said is wide enough for the miners to be hoisted through.

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Although no drill has been found, archaeologists have discovered beads of the same 2.5mm diameter as the holes found in the teeth, indicating the people did have the capacity to do delicate work.

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The drill would use one of the three bore holes already made as a pilot and widen the diameter to about 28 inches, which officials have said is wide enough for the miners to be hoisted through.

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