Not the commenter above, but as a kid I knew about slavery, and about "people owning other people". I even knew about how long it had gone on for, and that slaves were transported in ships around the Mediterranean.

That said, I was wholly unprepared for the brutality of the Atlantic slave trade. Due to the higher round trip times, and lack of mid-trip provisioning, there was a requirement to be more "efficient" than the slave trade across land or sea previously.