Question: What event in 1170 may have caused the creation of 5,000 km2 large shallow bay of the North Sea in the northwest of the Netherlands?
Sentence (from "Zuiderzee"): 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.
Relevant evidence: No