Every measurement starts at zero. This simple fact is anything but trivial when it comes to measuring geographic elevations, as opinions have long differed about how mean sea level should be defined. Sea level as a reference point changes not only with the tides but also differs depending on the location at which it is measured. The fact that global warming is causing changes in sea levels does not make matters any easier. At the Max Planck Institute for the History of Science, Wilko Hardenberg focuses on this variable reference and its history.