From the 8th/9th centuries onwards the former peat land area in the northwestern part of the Netherlands was drained and reclaimed. Drainage, peat digging and marine erosion of peat deposits created an extension of shallow marine habitats into which Zostera marina (Eel-grass) expanded. Modern publications do not note the use of Eel-grass as starting material for salt-making. Archaeological evidence, however, indicates salt-making activities by using Zostera in medieval times. We postulate that, after salt-containing peat had become difficult to obtain, Eel-grass was used for salt production.