Ya'Seem was a site of major archaeological interest.
Its discovery by M'Tell was considered one of the greatest archaeological discoveries ever, comparable to Schliemann finding Troy. (TNG: "The Chase")
This location was only mentioned in dialogue.
It was not specified what exactly Ya'Seem was, although the mention of M'Tell first stepping on it suggests it was something to be landed on.
According to the script, the pronunciation for Ya'Seem was "ya-SEEM".