A 10,000 year old wall submurged.

Germany: Ice-age stone wall found under Baltic Sea

https://www.dw.com/en/germany-ice-age-stone-wall-found-under-baltic-sea/a-68243132

8 hours ago8 hours ago

Researchers believe they have found the oldest man-made structure in the Baltic Sea region. The wall is believed to be more than 10,000 years old.