Which preposition to use with underline

in Occurrences 4%

The party were in a cave, and yet we can hardly call it a cave; it appeared to be merely a huge underline in the side of the cliff, as it was open, as the guide said, facing Creedon Street.

of Occurrences 1%

UNDERLINEThe underline of the head, viewed in profile, runs almost in a straight line from the corner of the lip to the corner of the jawbone, allowing for the fold of the lip, but with no loose skin to hang down.

Which preposition to use with  underline