The lower surface or part of something.

“The underneath of the aircraft was painted blue.”

A background radio sound track played during a specific announcement or program.

Under, lower.

“You can have the underneath bunk.”

Below; in a place beneath.

On the underside or lower face.

Under, below, beneath.

“Underneath the water, all was calm.”

Under the control or power of.

“There was little freedom underneath the jackboot.”