We have a woodburning fireplace we are converting to gas logs. I am trying to
drill a hole to accommodate the gas pipe through the floor in a front corner of
the firebox to a utility space under the house. I get through the firebrick,
about 3 1/2", and then hit something my carbide bit cannot penetrate.

Does anyone have a suggestion how to detect what I'm hitting? Do they use steel
plate reinforcement in fireplace construction? Any help is appreciated.