In all, people found the Americas at least seven different times. For at least six of those, it wasn't so new after all. The discoverers came by sea and by land, bringing new genes, new languages, new technologies. Some stayed, explored, and built empires. Others went home, and left few hints they'd ever been there.

From last to first, here's the story of how we discovered the Americas.

7. Christopher Columbus: AD 1492

In 1492, Europeans could reach Asia by the Silk Road, or by sailing the Cape Route around the southern tip of Africa. Sailing west from Europe was thought to be impossible.

The ancient Greeks had accurately calculated that the circumference of the Earth was 40,000 km, which put Asia far to the west. But Columbus botched his calculations. An error in unit conversion gave him a circumference of just 30,000 km.

This mistake, with other assumptions born of wishful thinking, gave a distance of just 4,500 km from Europe to Japan. The actual distance is almost 20,000 kilometers.

So Columbus's ships set sail without enough supplies to reach Asia. Fortunately for him, he hit the Americas. Columbus, thinking he'd found the East Indies, called its people "Indios," or Indians. He ultimately died without realizing his mistake. It was the navigator Amerigo Vespucci who realized Columbus had found an unknown land and in 1507 the name America was applied in Vespucci's honor.