The Middle Passage was a part of a triangular trade that was going on at the time, the link between Africa and the West Indies then on to America. The triangle itself consisted of the Americas, Europe, Africa, and the West Indies; all traded different things that the other needed resulting in a continuous trade route that made a triangular shape--hence its name. Americas and Europe traded back and forth, America sending raw materials like lumber, fish, and tobacco in return for teas, spices, cloth, tools, and other ready-made things; the connection between Europe and Africa was all iron products as well as some rum from the Americas; Africa sent out slaves and gold to the West Indies who, from there, sent slaves, sugar, and molasses to America. This trade lasted for hundreds of years, and the Middle Passage was the most grueling route of them all.
Slaves were usually acquired unwillingly through raids or others being sold into it and often had no clue what await them across the seas. They were packed the boats, cramped into the underbelly of the ships and shackled together. Many starved or died from disease that ran rampant in these filthy living conditions, and many commit suicide, figuring an eternal sleep would be better than knowing they might never see freedom again. It wasn't just the slaves that were dying either, many white crewman would suffer the same fate; then again how could one not when death was a constant threat as disease, murder, starvation, suicide, asphyxiation, and severe depression rapidly claimed the lives of so many. And this was just the passage from one place to the other, often the conditions were equal, if not worse for the slaves when--if--they reached America.
It was awful, inhumane, immoral, and yet the Middle Passage was extremely lucrative; it was all about the trade and what one had in these days, and