The Normaal Amsterdams Peil, or Amsterdam Ordnance Datum, is a water level indication that was set up in the 17th century.
Later it was adopted by other European countries.

The educational display in the hall of the City Hall shows the current water level, and shows a bit more about water levels in the Netherlands.

Address:
Amstel 1, Amsterdam
Web:
http://en.wikipedia.org/wiki/Amsterdam_Ordnance_Datum

Opening hours:
Office hours.
Entrance fee:
Access is free.