NAP stands for Normaal Amsterdams Peil or the normal water level in Amsterdam, which is slightly lower than sea level. NAP is used as a base to measure how high or low water levels are. So when the river Rhine is high, it is described as a certain number of metres ‘above NAP’. The lowest point in the Netherlands, in Nieuwerkerk aan den IJssel, is 6.76 metres below NAP. Scary.