Before the 1400s, the 'Known World' was limited to Europe, parts of Africa, and parts of Asia. Most people believed the world was smaller, and flatter, than it actually is. Most sea travel was limited to the coasts, for many that ventured farther did not return. Transportation over land was often dangerous and tedious, but despite that, explorers of the Middle Ages, like Marco Polo, made their way into Asia and opened trade routes. Other boundaries were blocked by Muslim territory or open ocean. Despite this, the Black Death in the 13th century limited trade and travel, and decimated the European population.