Height above mean sea level is a reference for a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level.
The zero level varies in different countries due to different reference points and historic measurement periods.
Climate change and other forces can cause sea levels and elevations to vary over time.
Land mass subsidence (as occurs naturally in some regions) can give the appearance of rising sea levels.
Conversely, markings on land masses that are uplifted (due to geological processes) can suggest a relative lowering of mean sea level.