Vikings Were in the Azores 600 Years Before the Portuguese

 But they didn't seem to have stayed long. We can tell from the mouse DNA. 

Once mice have established somewhere, they can find ways to survive on their own even after the humans are gone.  But they can only get to islands with humans on ships, and probably only in the context of livestock, whose food they sneak off with. The Azores are about a third of the way across the Atlantic from Portugal to New York, and the historical record has always been that the Portuguese were the first to get there in the 1400s. Norse scholars have searched in vain for a reference to Viking travels that could possibly be the Azores. The Vikings did go an amazing number of places, after all.

But help has come from an unexpected corner, the biologists. When studying mouse DNA to determine what percentage and which islands had mice with origins in Portugal and which from Madeira (which have earlier been shown to be ultimately Danish, meaning that the Danes did at least get there.) They found on three small islands mice that had the same mtDNA (female line, very stable generation after generation) as mice from Norway and Iceland - and not all that closely related to the Danish ones, which they split from far earlier. 

Once they knew that someone from Norway had arrived with mice sometime, they were able to look for other clues, and found them, as the article tells us. Charcoal from intentional burning is a big clue everywhere. That can be radiocarbon dated, and confirms the most likely suspicion: the years when Vikings sailed everywhere, and traded with everyone, largely slaves. Vikings brough a lot of stuff on their ships for exactly this reason. Sometimes you get blown off course and have to survive in a place for a while until you can get back home.  

They must have been very good at getting back home.