The travels of Portuguese traders to western Africa also acquainted the Portuguese with the African slave trade, already widely in practice in West Africa and funded by sugar production on the newly colonized Atlantic islands. Upon discovering the immense global market for sugar, the Portuguese began to trade enslaved people across the Atlantic to toil on the sugar plantations. The Portuguese fort Elmina Castle, located in modern-day Ghana, became more of a holding pen for enslaved Africans from the interior of the continent than a trading post, as the markets for slave labor in both Europe and then the New World boomed. Portuguese colonization in the 1400s inaugurated an era of aggressive European expansion across the Atlantic. The Spanish, threatened by the Portuguese monopoly on enslaved Africans and expansion in the Atlantic, started their own colonization project with Christopher Columbus in 1492. The competition between the two nations continued and drew more and more Europeans to the New World.