(CNN)"In 1492, Columbus sailed the ocean blue."
That rhyme has long been how American students were introduced to Christopher Columbus in elementary school.
Students are taught that Columbus is the one who discovered the Americas, sailing across the Atlantic in his three ships: The Niña, Pinta and Santa Maria. The Italian explorer is even celebrated every October during a federal holiday named after him.
But the man credited for discovering the "New World" has long been considered a contentious figure in US history for his treatment of the indigenous communities he encountered and for his role in the violent colonization at their expense.
Dozens of cities and states -- such as Minnesota, Alaska, Vermont and Oregon -- have already replaced Columbus Day with Indigenous People's Day.
Now, in response to the nationwide protests and conversation surrounding racial inequality, people have been tearing down statues of Columbus to bring awareness to the cruelty he brought upon indigenous people.
So what did Columbus really do and why is he being labeled as a "tyrant" rather than the hero we were taught to believe?
"We should question why we as Americans continue to celebrate him without knowing the true history of his legacy, and why a holiday was created in the first place," Dr. Leo Killsback, a citizen of the Northern Cheyenne Nation and assistant professor of American Indian Studies at Arizona State University, told CNN in 2016.
He wasn't the first to discover the Americas
There's no doubt that Columbus' voyages had an "undeniable historical impact, sparking the great age of Atlantic exploration, trade and eventually colonization by Europeans," according to historian David M. Perry, who wrote an op-ed for CNN about Columbus Day in 2015.
But Columbus wasn't the first to discover the New World. Indigenous people had been living there for centuries by the time he arrived in 1492 and Leif Eriksson and the Vikings beat him to it five centuries earlier.
He enslaved the natives
During his voyages through the Caribbean islands and the Central and South American coasts, Columbus came upon indigenous people that he labeled "Indians."
Columbus and his men enslaved many of these native people and treated them with extreme violence and brutality, according to History.com.
Throughout his years in the Americas, Columbus forced natives to work for the sake of profits. Later, he sent thousands of Taino "Indians" to Spain to be sold, and many of them died during the journey. The natives who weren't sold into slavery were forced to look for gold in mines and work on plantations.
While he was governor of what is now the Dominican Republic, Columbus killed many natives in response to their revolt, according to History.com. To prevent further rebellion, he would have the dead bodies paraded through the streets.
He brought new diseases
The indigenous societies of the Americas "were decimated by exposure to Old World diseases, crumbling under the weight of epidemic," Perry wrote in his CNN op-ed.
The Taino population weren't immune to diseases such as smallpox, measles and influenza, which were brought to their island of Hispaniola by Columbus and his men. In 1492, there were an estimated 250,000 indigenous people in Hispaniola, but by 1517, only 14,000 remained, according to the Oklahoma Medical Research Foundation.
Some historians believe that the impact of European and African settlers in the New World possibly killed as much as 90% of the native populations and was deadlier than the Black Death was in medieval Europe, OMRF said.