sea level 
noun
The level of the sea’s surface, used in reckoning the height of geographical features such as hills and as a barometric standard: it is only 500 feet above sea level...Compare with mean sea level.
- The South Pole Observatory was established at the geographical south pole at 2837 m above sea level in 1957 as part of the International Geophysical Year.
- Considering that Amsterdam is situated two meters below sea level, the Dutch have really done a magnificent job of incorporating nature into the function and aesthetics of the city.