The Zuiderzee or Zuider Zee (/ˌzaɪdər ˈziː, - ˈzeɪ/ ZY-dər ZEE, -⁠ ZAY, also US: /ˌzɔɪdər ˈzeɪ/ ZOY-dər ZAY, Dutch: [ˌzœydərˈzeː] ; old spelling Zuyderzee or Zuyder Zee) was a shallow bay of the North Sea in the northwest of the Netherlands, extending about 100 km (60 miles) inland and at most 50 km (30 miles) wide, with an overall depth of about 4 to 5 metres (13–16 feet) and a coastline of about 300 km (200 miles). It covered 5,000 km2 (1,900 sq mi). Its name is Dutch for "southern sea", indicating that the name originates in Friesland, to the north of the Zuiderzee (cf. North Sea). In the 20th century the majority of the Zuiderzee was closed off from the North Sea by the construction of the Afsluitdijk, leaving the mouth of the inlet to become part of the Wadden Sea. The salt water in