The information we get from satellites is essential for understanding how fast our seas are rising so that decision-makers are equipped to take appropriate mitigating action. Satellites carrying altimeter instruments systematically measure the height of the sea surface so that sea-level rise can be closely monitored. Altimetry measurements over the last 25 years show that, on average, sea-level is rising about 3 mm a year, and this rise is accelerating. However, this average number masks big differences as it is not rising at the same rate everywhere. In fact, while in some places it is rising as much as 10 mm a year, in other places it is falling by 10 mm a year. There are many reasons for this imbalance such as differences in local gravity, freshwater discharge from rivers and differences in seawater density.