The mystery began on August 18, 1227, when Mongol leader Genghis Khan died of unknown causes while leading a military campaign in China. According to legend, Khan’s successors killed anyone who witnessed his funeral procession on its way back to the Mongol capital of Karakorum. Some 800 soldiers are said to have massacred the 2,000 people who attended his funeral, before being summarily executed themselves. Khan’s corpse was then placed in an unmarked grave to ensure his rest would be undisturbed. Horses trampled all evidence of the burial, and some say a river was diverted to flow over the site. As a result of these extreme measures, the location of Khan’s tomb has remained unknown for almost 900 years.

Most experts believe Khan was buried somewhere near his birthplace in Khentii Aimag, northeastern Mongolia, and that his descendants may be buried there along with him—but they don’t have much more to go on than that. Researchers weren’t even allowed in the area until after the Soviet occupation of Mongolia ended in the 1990s. And in the decades since, various groups have been pressured to give up their searches due to protests from the Mongolian government and public that excavation would disturb the rest of their national hero.

Such opposition has not halted the hunt. In 2004 Japanese-Mongolian researchers discovered the remains of what they think is Khan’s palace complex on the grassy steppe of Khentii Province, 150 miles east of the Mongolian capital, Ulan Bator; they believe his tomb may be somewhere nearby. And since 2008, the Valley of the Khans Project has been using cutting-edge technology to search for Khan’s final resting place. The project has enlisted thousands of “citizen scientists” to comb through high-resolution satellite images of the region looking for possible clues, giving amateurs with a home computer and an Internet connection a rare chance to help solve one of history’s most enduring riddles.