How Industrial-Scale Tar Production Powered the Viking Age

"There were two kinds of Tar Pits: Mounds with ducts and underground funnel-shapes with container in bottom": https://fi.wikipedia.org/wiki/Tervahauta

But then again the Uppland area was populated with Finnish people pre 1000AD.

Swedish scientist make these amazing discoveries frequently. One was rune-scroll with "magical language of unknown origin". When read phonetically at some convention, some random visitor from Finland recognized the language.