1. [adj]  not made to sound; "the silent `h' at the beginning of `honor'"; "in French certain letters are often unsounded"
2. [adj]  situated at or extending to great depth; too deep to have been sounded or plumbed; "the profound depths of the sea"; "the dark unfathomed caves of ocean"-Thomas Gray; "unplumbed depths of the sea"; "remote and unsounded caverns"

UNSOUNDED is a 9 letter word that starts with U.