thumb|right|300px|Historical map of the Netherlands (1658) with the Zuyder Zee
The Zuiderzee or Zuider Zee (; old spelling Zuyderzee or Zuyder Zee), historically called Lake Almere and Lake Flevo, was a shallow bay of the North Sea in the northwest of the Netherlands. It extended about inland and at most wide, with an overall depth of about and a coastline of about . It covered . Its name is Dutch for "southern sea", indicating that the name originates in Friesland, to the north of the Zuiderzee (cf. North Sea).
It is generally acknowledged that the Zuiderzee existed from roughly 1170, following the devastating All Saints' Flood, until 1932, when the Afsluitdijk was completed. The majority of the Zuiderzee was closed off from the North Sea, leaving the mouth of the inlet to become part of the Wadden Sea. The salt water inlet changed into a fresh water lake now called the IJsselmeer (IJssel Lake) after the river that drains into it, and by means of drainage and polders, an area of some was reclaimed as land. This land eventually became the province of Flevoland. Part of the IJsselmeer was also divided into the Markermeer.
History
Lake Flevo
thumb|The region of the [[Netherlands 500 BC.]]
thumb|The region of the [[Netherlands 50 AD.]]
In classical times the body of water at this location was called Lake Flevo (Flevo Lacus This was the central and largest lake in a region filled with a mixture of lowland and freshwater lakes occupying the area later filled by the Zuiderzee. It was separated from the sea by a belt of marsh and fen; at that time, the coastline ran along the line of the Frisian Islands. A number of streams, including the Vecht, Eem, and Ysel, fed into the lake. The lake itself fed out into the North Sea through the Vlie (). It existed in Roman times and the early Middle Ages.
From the Indo-European root *plew- "flow", the name was transmitted by the Roman geographer Pomponius Mela in describing this region. In his treatise on geography of 44 AD, Pomponius speaks of a . He writes: "The northern branch of the Rhine widens as Lake Flevo, and encloses an island of the same name, and then as a normal river flows to the sea". Other sources rather speak of Flevum, which could be related to today's Vlie (Vliestroom), i.e. the seaway between the Dutch islands of Vlieland and Terschelling. This last name is grammatically more probable for a geographical indication, which is why it is assumed that Pomponius confused the declension of the word giving the name Flevo. In fact the Vlie formed outfall from the lake into the North Sea.
In the second half of the twentieth century the Flevopolders and a new province, Flevoland, took the name of the body of water which lay there long ago.
Over time these lakes gradually eroded their soft peat shores and spread (a process known as waterwolf). Some part of this area of water was later called the Vlie; it probably flowed into the sea through what is now the Vliestroom channel between the islands of Vlieland and Terschelling. The disaster marked the rise of Amsterdam on the southwestern end of the bay, since traders from the Hanseatic cities could now reach the area.
Zuiderzee
The formation of the Zuiderzee was a gradual process, influenced by several storm surges in 1163/64, 1170, 1173, and especially the of 1196, which completed much of what the earlier floods had set in motion.
Zuiderzee Works
thumb|Map of the [[Zuiderzee Works in the Netherlands]]
thumb|Satellite view of the modern region, with parts of the Zuiderzee Works annotated
The construction between 1927 and 1932 of a 19-mile long dam (the Afsluitdijk), under plans originating from Cornelius Lely, enclosed the Zuiderzee. The creation of this dam was hastened by the Flood of January 1916. Plans for closing the Zuiderzee had been made over thirty years earlier but had not yet passed in parliament. With the completion of the Afsluitdijk in 1932, the Zuiderzee became the IJsselmeer, and the outer portion of the Zuiderzee became the Wadden Sea. Large areas of land, mainly for agricultural use, were subsequently reclaimed from the water through the construction of polders with dams, pumping, and other hydrological technology. Four had been built by the early 1980s. They were the Wieringermeer Polder built in 1930; the Northeast (Noordoost) Polder, built in 1942; the East (Oostelijk) Flevoland Polder, built in 1957; and the South (Zuidelijk) Flevoland Polder, completed in 1968. A fifth, Markerwaard, began construction in 1963, and became partially complete, but was abandoned in the mid-1980s.
Geography
Around the Zuiderzee many fishing villages grew up and several developed into walled towns with extensive trade connections, in particular Kampen, a town in Overijssel, and later also towns in Holland such as Amsterdam, Hoorn, and Enkhuizen. These towns traded at first with ports on the Baltic Sea, in England, and in the Hanseatic League, but later also with the rest of the world when the Netherlands established its colonial empire. When that lucrative trade diminished, most of the towns fell back on fishing and some industry until the 20th century when tourism became the major source of income.
Contained within the Zuiderzee were five small islands,