Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Romans 1:21
"A total of 199 new skulls have been found in centuries-old Aztec "tower of skulls". 
These skulls are being assumed to be of captured warriors from the enemies' armies, and the tower is being understood to be a warning to the rivals of the Aztec empire.
While it is clear the skulls are a mix of men and women, it is being assumed some of them were simply for making sacrifices.
It is located in the area of the Templo Mayor, one of the main temples of the Aztec capital Tenochtitlan in the historic district of modern-day Mexico City.
The sacrifice of these skulls, it is believed, was being made to ensure existence of the universe due to which it is often known as the tower of life, rather than death." WION
