Humans have a long history of slave trading, often over vast distances, but nothing has rivaled the Atlantic slave trade in size. Between the early 1500s and the 1860s, slave traders forced some 12.5 million men, women, and children aboard transatlantic slave ships on Africa’s shores. This number does not include the millions more who died during the journey from the interior to the coast or who perished before a slave ship arrived to carry them away.

Most of us learned in school that slave ships followed a triangular route from Europe, Africa, the Americas and back to Europe. But major variations existed. Thousands of voyages began in the Americas, continued to Africa and returned to the Americas. The Rio de Janeiro (Brazil) – Luanda (Angola) – Rio de Janeiro journey formed the largest single route in the entire slave trade. Other voyages originated in U.S. ports like Newport, Rhode Island and Charleston, South Carolina, and brought captives to the Americas on their return.

While most slave traders were men, hundreds of women invested in the trade, according to slavevoyages.org. In Britain, France and the Netherlands, widows of slave-trading husbands commonly took over their investments. One of the last-ever slave traders, American Mary Watson, dispatched her own ships from New York in the 1850s and early 1860s. After Abraham Lincoln’s administration shut down the slave trade in the U.S., Watson attempted to establish a new base in Spain in 1862 but died of tuberculosis.

Captives endured tremendous violence and trauma during the Atlantic crossing. According to slavevoyages.org, an estimated 15 percent died. Slave traders did everything in their power to prevent their captives from fighting back, arming themselves and shackling their human “cargo.” But captives resisted. Rebellions occurred on at least 10 percent of voyages, usually when ships were on or near the African coast. While they rarely succeeded, these revolts had a broader impact. By forcing slave traders to take extra precautions like increasing the number of sailors on board ships, rebels drove slave traders’ costs higher and reduced the number of voyages put to sea. By some scholars’ estimates, this prevented about 1 million captives from enduring the Middle Passage.

Books and movies often depict the antislavery movement as being led by political leaders like Britain’s William Wilberforce. While such men played vital roles in the abolition of the slave trade, its earliest and most consistent opponents were enslaved people themselves. Their opposition proved crucial, as evidenced by the revolt that took place on the French Caribbean colony of Saint Domingue, one of the largest destinations for slave ships in the 1700s. In the 1790s, enslaved people there rebelled against the institution of slavery and later against French colonial rule. And they won. By 1804, they had defeated France (and its leader Napoleon Bonaparte), and ensured that the slave trade and slavery itself would never be resurrected. They renamed their country Haiti, the name by which we know the country today.

By the late 1700s, many countries questioned the legitimacy of the slave trade. By the 1830s, every slave-trading nation in Europe and the Americas banned the traffic, either through international treaties or via their own national laws. But many powers, including Brazil and Spain, did so reluctantly and under diplomatic pressure. They had little intention of seriously enforcing their bans. The slave trade continued to produce profits—in fact, even more so in the 1800s—and traffickers paid large bribes to government officials to ignore their crimes. Most of the captives who endured the illegal slave trade in the 1800s went to Brazil and the Spanish island of Cuba. Many were transported under the American flag and on U.S.-built ships, which the U.S. authorities had little interested in policing. In total, slave traders smuggled 1.65 million captives aboard illegal slave ships in the 1800s—13 percent of the entire slave trade, according to slavevoyage.org. The last slave ship to cross the Atlantic Ocean arrived in Cuba in the late 1860s.

The Atlantic slave trade rippled out far beyond the Atlantic Ocean. European slave traders sourced fabrics from South Asia and cowrie shells from the Maldives in the Indian Ocean to trade for enslaved Africans. Parts of non-Atlantic Africa also participated in the slave trade. Some slave ships carried captives from Mozambique on Africa’s eastern coast to the Americas. In some parts of the Americas, enslaved people mined metals like silver and gold, which slave owners sometimes traded to China.

The slave trade radically changed lives, economies, environments and cultures. First, of course, it devastated millions of enslaved people. In addition to the 1 to 2 million who died during the Middle Passage, survivors usually faced lifelong enslavement and hard labor in the Americas.

The slave trade also reinforced racial hierarchies in the Americas and Europe that upheld the oppression of Black people for centuries. Closely intertwined with capitalism, industrialization and imperialism, the trade strengthened those economic and political systems, enriching enslavers and societies on both sides of the Atlantic. And it transformed ecosystems and environments, transporting invasive species like mosquitoes from Africa to the Americas and forcing enslaved people to clear trees to cultivate cash crops like sugar and cotton for global markets.

On a cultural level, enslaved Africans made a huge mark on the Americas—unsurprising given the richness and extraordinary diversity of African cultures and the fact that more enslaved Africans crossed the Atlantic Ocean than Europeans until the 1820s. African influences permeate language, music, religion, food and medicine across the Americas.

Historians continue to gather information about captives and to tell their stories. New databases such as slavevoyages.org/past/database and enslaved.org/ contain names and other information about tens of thousands of enslaved people. One individual who has received lots of attention recently is Oluale Kossola (later known by the name Cudjo Lewis), who has a remarkable story. Captured in 1860 by Dahomey warriors in modern-day Benin and brought to Alabama in 1860, Kossola journeyed aboard the Clotilda, the last slave ship to arrive in the United States. Kossola survived the Middle Passage and slavery in the American South before joining with fellow Clotilda captives and forming a new settlement called Africatown, Alabama. This town still exists today.