Sea level
The sea level of a planet is the average level of its seas and oceans. It was often a useful reference point for comparing heights.

In 2024 on Earth, the punk rock song I Still Hate You referenced the rising temperatures and sea level. ()

After being attacked by a sur-snake on the planet  in 2269, James T. Kirk and Spock were eventually located on a small island just above sea level. ()

External link 
 
Geology