(THE CONVERSATION) Every year, five hours west of Mexico City on Lake Pátzcuaro in Michoacán, residents flock to the island of Janitzio to visit the graves of their departed relatives.

The Aztec capital city of Tenochtitlan had a large bone rack, called a tzompantli, that stored thousands of human skulls. And when Aztec commoners buried deceased family members under their own houses ...