Dynasties and empires rose and fell along the Wei He River valley, where Xi'an lies. While the emperors are gone, their legacy awaits the spades and brushes of archaeologists exploring this crucible of Chinese history and culture. The terracotta warriors, one of archaeology's greatest accidental finds, hint at what else could lie under the barely scratched fields where emperors and aristocrats lie interred beneath 500 burial mounds. These tombs rise out of a fertile plain where orchards, renowned throughout Shaanxi for their crispy apples along with maize and other vegetables, form a pleasing agricultural mosaic. Come winter, the region takes on a starker beauty, eerie when dense fogs descend, shrouding the tombs.Burial mounds, tombs, pyramids, call them what you will - they squat in other corners of Asia as well as the Middle East, Europe and Latin America. While some of the Wei He valley tombs bear comparison with the long barrows found around England's Salisbury Plain and elsewhere in northern Europe, a few investigators draw more astonishing connections. Hartwig Hausdorf, a German writer who researches unexplained phenomena, suspects the involvement of aliens. Black Africans played a crucial role in their construction, suggests historian Legrand Clegg, who points to evidence of a negroid people in China. There is also a theory that the Xi'an mounds map the Gemini constellation as it would have appeared on the horizon at spring equinox, 10500 BC. Coincidentally (or perhaps not?) Egypt's Great Pyramid complex was built to map Orion's position in the skies in 10500 BC, or so says British expert on unexplained phenomena Graham Hancock.