How did humans successfully migrate across huge oceans? Why did they do choose to?

by bsa86

This question might be difficult to answer, especially the second part, but I'm curious about how humans managed to successfully travel to unknown continents and islands, particularly Greenland, Oceania, Madagascar, and Polynesia. Were they merely the successful few to find land?

How did they create boats that were capable of travelling huge distances, and how did they survive whilst on the boats?

Furthermore, I can't think what would inspire people to travel into the unknown ocean without any way of knowing that they'd find land - is there any knowledge of why they migrated?

Searocksandtrees