Columbus' voyages also had a huge, largely negative impact on the people the Spanish encountered in the Americas. And excitingly from my perspective, once Columbus returned from San Salvador, we can speak for the first time of a truly world history. Except for you, Australia. So who was the greatest mariner of the 15th century? Well, as usual, it depends on your definition of greatness. If you value administrative competence over ill-advised adventure, then Zheng He is certainly the winner. But the reason we remember Columbus over him or Vasco de Gama is that Columbus' voyages had a lasting impact on the world, even if it wasn't necessarily a positive one.