A word frequently translated as the grave is the Hebrew שְׁא֣וֹל – sheol, but sheol is far more than just the grave. The grave is a portal into the underworld realm of the dead.
Hell with Benefits
There are benefits to hell being at the center of the Earth. It solves the problem of exponential nuclear decay and renders radiometric dating useless for the development of an ancient Earth narrative.
The Hebrews had a well developed idea of life after death. For them death, the grave, was a portal into a complex underworld realm. The grave is just the beginning of what happens next.