Man descending a tunnel to Sima de los Huesos. The route to Sima de los Huesos (Pit of Bones) covers more than 600 metres of difficult passages, some of it requiring researchers to crawl on their stomachs, followed by this vertical shaft some 13 metres deep leading down to a pitch black, cool chamber. The recovery of at least 32 individuals suggests that this site was used as a cemetary. Sima de los Huesos is one of several sites in Sierra de Atapuerca, Spain, that have been studied over the past century, leading to a massive advancement in the understanding of human evolution. In 2000, Sierra de Atapuerca was made a world heritage site by UNESCO.