The “Midnight Terror Cave” is located to the south of Belmopan near the Mennonite community of Springfield in the Cayo District of Belize. It is an extensive subterranean complex that archaeologists theorize was used for a variety of ritual purposes including human sacrifice – this last theory based a the huge quantity of human remain found, including roughly 9000 bones. Based on studies of teeth found a large proportion of the individuals found were children, some coming from 200 miles away. Based on study of the minerals found in these remains it appears that none of these people came from Belize.
Here’s an account from the Washington Post: http://www.washingtonpost.com/news/morning-mix/wp/2015/03/30/why-this-mysterious-mayan-cave-full-of-childrens-bones-may-be-evidence-of-ancient-human-trafficking/
Here’s details of the survey of skeletal materials made by Cal State’s Anthropology Department: http://web.calstatela.edu/academic/anthro/MidnightTerror.htm
and here’s a copy of the academic paper: “Preliminary Observations on the Investigation of Midnight Terror Cave Belize”