Complex sentence: The Norsemen, the Portuguese and the Spanish were the first to explore and to cross it systematically, from northeast (Europe) to its islands and the Americas, and from the North Atlantic to the South Atlantic.
Split into simpler sentences: The Norsemen, the Portuguese and the Spanish were the first to explore. The Spanish were the first to cross it systematically, from Europe to the Americas, specifically from the North Atlantic to the South Atlantic.