The All Saints' Flood of 1170 (Allerheiligenvloed) was a catastrophic flood in the Netherlands

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 that took place in 1170. Large parts of the Northern Netherlands, and Holland territories were overflowed. This is the flood that created the Zuiderzee. "Lake Flevo" was once a fresh water lake, but after this flood a sea channel opened a connection from the North Sea

North Sea

In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

 into the lake through 'Creiler Forest', and turned the lake into the salt-water sea known as the Zuiderzee. The Creiler Forest vanished forever under the waves.

The sea area increased inside the Netherlands and large peat

Peat

Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

areas developed, which were easily washed away.