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 "All Saints' Flood (1170)"): "Lake Flevo" was once a fresh water lake, but after this flood a sea channel opened a connection from the North Sea into the lake through Creil Woods, and turned the lake into the salt-water sea known as the Zuiderzee.
Relevant evidence: No