[AdmSmith]

Here are three:

Jomsborg or Jómsborg (German: Jomsburg) was a semi-legendary Viking stronghold at the southern coast of the Baltic Sea (medieval Wendland, modern Pomerania), that existed between the 960s and 1043. Its inhabitants were known as Jomsvikings. Jomsborg’s exact location, or its existence, has not yet been established, though it is often maintained that Jomsborg was located on the eastern outlet of the Oder river. Historian Lauritz Weibull dismissed Jomsborg as a legend.

https://en.wikipedia.org/wiki/Jomsborg

Rungholt was a settlement in North Frisia, in what was then the Danish Duchy of Schleswig. The area today lies in Germany. Rungholt reportedly sank beneath the waves of the North Sea when a storm tide (known as Grote Mandrenke or Den Store Manddrukning) hit the coast on 15 or 16 January 1362. The exact location of Rungholt has yet to be conclusively identified.

https://en.wikipedia.org/wiki/Rungholt

Noreia is an ancient lost city in the Eastern Alps, most likely in southern Austria. While according to Julius Caesar it is known to have been the capital of the Celtic kingdom of Noricum, it was already referred to as a lost city by Pliny the Elder (AD 23 – AD 79). The location of Noreia has not been verified by modern researchers.

https://en.wikipedia.org/wiki/Noreia

[SunkenCiv]

Thanks! I can't seem to find it, but there's a topic about an earlier flood / tsunami cited anecdotally by a Roman source that struck the North Sea coastline of mainland Europe.

• Tsunamis in the Alps? A killer wave slammed medieval Geneva, a new study says... [500 A.D.] [11/01/2012]
• 10 Mysterious Underwater Cities You Haven't Heard Of [12/14/2014]
• Lost since 1362: Researchers discover the church of a sunken medieval trading place [05/28/2023]
• Archaeologist suggests location of legendary Viking settlement of Jomsborg [Heritage Daily, May 30, 2023]
• neapolis [keyword]