Europeans in the Middle Ages firmly believed that there were only 3 continents: Europe, Asia and Africa. Since European sailors lacked navigational instruments for sailing in deep sea waters, they relied on sailing at such a distance they always kept the sight of the shoreline to get their bearings, know their position at all times and avoiding getting lost. The only medieval sailors able to sail in deep sea water were the Vikings who in the tenth and eleventh centuries sailed across the Atlantic far beyond the sight of any known shoreline aided by the observation of the position of the sun and stars in the sky and the changes of color in the ocean water. This is how they were able to settle in Iceland and Greenland, and even found a few colonies in North America for a few years. Unfortunately, as they converted to Christianity, this knowledge became lost.
Medieval Europe's economy became very active as a result of the Crusades and their contact with the Muslim world. They learned about so many exotic and great products sold in the Middle East markets, so international trade became a fast and great way for European merchants to become rich, first in the Italian cities of Genoa and Venice, and then in the northern cities of the Netherlands and England. The maritime routes in the Mediterranean Sea they used to reach the Middle East had been laid down some 2,000 years earlier by ancient Phoenicians, Greeks and Romans, so European sailors were not worried about losing sight from the coastline. As the years went by, most of the markets of the Middle East were denied to the Europeans, and the only market left for trading in the most coveted and expensive goods of the time, spices, was in the city of Constantinople. In 1453, Constantinople was captured by the Ottoman Turks, fiercely opposed to Western Christians, and the spice trade ceased.
At the time, Portugal, a European country off the Atlantic Ocean, had just become independent from the Arabs, and one of his rulers, Henry (latero on nicknamed "The Navigator"), came up with the idea of reaching India and China, the main spice producers in the world, sailing around the African coastline,avoiding the Ottomans' opposition or the need of depending on an Arabic middleman. This initiated a number of expeditions aimed at exploring new trade routes motivated by the chances of resuming the spice trade in Europe and for the sponsors and investors who funded the expeditions, at first, and the trade voyages later, there was a lot of money to be made. Because of the potentially huge amount of money involved, the Portuguese claimed the routes around Africa to India and China as their exclusive property, the maps drawn as private and secret, so if any other country wished to reach India and China, a different route had to be explored and used. This made Columbus find a route to a continent unheard of at the time, America, by choosing a completely different route sailing westwards across the Atlantic, beyond the sight of the coastline (remember: these men were looking forward to make huge amounts of money in case they succeeded in reaching India and China by sea). Another Spanish expedition led by Portuguese captain, Ferdinand Magellan, circumnavigated (sailed around) the world for the first time in 1521.
A number of exploration expeditions would be made in the centuries to follow, but the most important expeditions that changed the world forever took place in the 1400s and 1500s.