Fireplace / Firebrick

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.
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there was a steel plate in the top of mine. can you look at some shavings/dust coming out of the hole to see what it is? reinforced concrete (you hit rebar)?
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A neighbor has a pencil magnet. I'll see what that tells me.
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There could be a steel pan filled with concrete under the firebox and in front of it. My gas pipe goes out the side and I think that is where gas logs and inserts expect it.

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We originally planned on going in the side, but through the floor is much more direct with no need to run a pipe on the outside wall of the house. Do you think it's possible/wise to continue drilling?

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With a hammer drill and the right bit you can do it for sure.

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You might want to use a slightly smaller bit with the hammer drill because holes usually come out large.

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This is Turtle.
I can be wrong here but a Carbide bit will cut a fire brick but when it comes to a regular brick. It has hell doing so. I think you just hit a real cured plain brick. Now a Steel rebar or steel plate can be it.
TURTLE
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