Fifty skulls and 250 jaw bones dating back more than 500 years have been discovered at what is thought to have been an Aztec sacrificial stone. It is believed they represent sacrifices to the Aztec god of death, Mictlantecuhtli. Per the 1st article:

Archaeologists have unearthed gruesome evidence of brutal Aztec rituals by uncovering 50 skulls and over 250 jaw bones at the Templo Mayor in Tenochtitlan (modern Mexico City). Found at one sacrificial stone below a ceremonial platform called the "cuauhxicalco," the human remains date back more than 500 years and represent the largest number of skulls ever found in one offering...."Usually people were killed on those stones by cutting open the chest and pulling out the heart," Barrera said.

Per the 2nd article:

Last August, the Mexican government announced experts had found an unprecedented human burial at another spot in the same temple in which the skeleton of a young woman, possibly sacrificed personifying a goddess, was surrounded by piles of nearly 1,800 bones....The skulls shown to the media Friday were in good condition but cracked on each side of the head, possibly because of the wooden stake that ran through them so they could be placed in a skull rack.

http://news.discovery.com/history/skulls-aztec-rituals-temple-12100...

http://www.usatoday.com/story/news/world/2012/10/05/mexico-aztec-sk...