HELDER, seaport in province of North Holland, 51 m. by rail N.N.W. of Amsterdam. Pop. (1926) 29,897. It is situated on the Marsdiep, the channel separating the island of Texel from the mainland and the main entrance to the Zuider Zee. It is the terminus of the North Holland canal from Amsterdam, and an important naval and military station. On the east side of the town is the harbour with naval wharves and magazines, wet and dry docks. The great sea-dike stretches from Nieuwe Diep to Fort Erfprins, a distance of about 5 m. It descends at an angle of 4o° for 200 ft. into the sea. The town is fortified and there is a per manent garrison, while the province can be flooded from this point. It has a meteorological observatory, a zoological station and a lighthouse..