There were various medieval legends about explorations (such as by St. Brendan). Even if those didn't happen, people in the 1400s might have thought they were real and acted accordingly.
A couple of Genoese explorers sailed west in 1291 and were never heard from again--did they make it across the Atlantic and get eaten by Caribs? Did they founder somewhere mid-ocean? No one knows what happened to them.
> A couple of Genoese explorers sailed west in 1291 and were never heard from again
Hey, pretty interesting:
Vandino and Ugolino Vivaldi
http://en.wikipedia.org/wiki/Vandino_and_Ugolino_Vivaldi