2
During the Migration Period (ca. 400-600s AD) large areas of Europe became depopulated or population sharply declined there. This includes Poland, there are theories about either depopulation or sharp decline but continuity in Poland in period 400-600s AD. This period is kind of a "black whole" in history and archeology.
Only in recent years, new discoveries revealed that some parts of Poland continued to be inhabited during the 500s.
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These maps are from a book published in 1998, before discoveries from the last two decades mentioned above:
1. Archaeological cultures in early 200s:
2. Archaeological cultures in late 300s:
Next two maps don't show archaeological cultures (because no "cultures" survived, only some scattered evidence of human activity), but just single findings:
3. Archaeological situation in late 400s:
4. Archaeological situation in early 600s:
Are there similar maps from other countries as well? AFAIK, East Germany (Elbe-Oder area) was depopulated even more than Poland during that period.
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Edit:
This map shows, more or less, the areas where new settlements radiocarbon-dated to the 500s have been discovered in recent years (after year 2000):
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