Question: When God created the earth it was good, so hell could not have been "in the heart of the earth" but was created after Adam's sin. Surely hell couldn't be in the heart of the earth in the sinless new creation. Where will the eternal Lake of Fire be located?
Response: We are not told the location of the Lake of Fire and waste our time and run the risk of getting into serious error when we speculate about things that the Holy Spirit has not seen fit to tell us in Scripture.
The inhabitants of hell, hades, or Sheol are disembodied spirits. Therefore, they have no physical form or "location," nor would that be required wherever they spend eternity. Why certain verses seem to "locate" hell somewhere in the heart of the earth is beyond my understanding.
This manner of speaking may be to show a contrast from "heaven," always depicted as above. It can't be "located" in space but in another dimension of existence beyond our present understanding.
Without speculating about what is beyond us, we accept the Bible by faith. Surely the Lake of Fire can't be in the new heavens and earth, where evil cannot enter.