Inmost
Inmost , adjective
[Old English innemest, Anglo-Saxon innemest, a double superlative form from inne within, from in in. The modern form is due to confusion with most. See In, and compare Aftermost, Foremost, Innermost.]
Deepest within; farthest from the surface or external part; innermost.
And pierce the inmost center of the earth.
The silent, slow, consuming fires,
Which on my inmost vitals prey.