Answer:
It is estimated that by the early 16th century as much as 10% of Lisbon's population was of African descent. After the European discovery of the American continent, the demand for African labour gradually grew, as other sources of labour - both European and American - were found to be insufficient.
Probably the ocean, seems interesting.
Deadly Deserts and Towering Mountains. ... China's towering mountain ranges cover two-thirds of the country, including the Himalayas, which hold the world's tallest peaks along the country's western borders. These treacherous deserts and mountains kept early China isolated from Western civilizations.
