Genghis Khan (c.1167-1227), Mongol conqueror: He became leader of his tribe, defeated other clans and was proclaimed Genghis Khan (Universal Ruler) of Mongol chieftains in 1206. Archive Photos/ Getty Images

The 1,748,000 refers to the estimated population in April 1221 of a Persian city called Nishapur. This city, located in what is now Iran, was a bustling cultural center during Khan's time. And during his campaign to the West, following his successful subduing of China, Nishapur was one of the cities his troops sacked.­

Genghis Khan (whose adopted name means "Universal Ruler" in Altaic, his native tongue) was something of a populist conqueror. He generally followed a self-imposed rule that those who surrendered to him were allowed to live. Common folk were often spared­, while their rulers usually were put to death. The same fate met anyone else who dared resist.



­In Nishapur, Khan's favorite son-in-law, Toquchar, was killed by an arrow shot by a Nishapuran. It's not entirely clear whether a revolt broke out after Khan's troops had already overtaken the city, or if the fateful event took place during an initial siege. Either way, this proved to be the death warrant for the inhabitants of the city.

Khan's daughter was heartbroken at the news of her husband's death, and requested that every last person in Nishapur be killed. Khan's troops, led by his youngest son, Tolui undertook the gruesome task. Women, children, infants, and even dogs and cats were all murdered. Worried that some of the inhabitants were wounded but still alive, Khan's daughter allegedly asked that each Nishapuran be beheaded, their skulls piled in pyramids. Ten days later, the pyramids were complete.

Exactly how many died at Nishapur during the siege is questionable, but it does appear that a great many people were killed and beheaded. There is no evidence that Genghis Khan was at the city when the massacre took place, however.

It's unclear why the legends say these events transpired in just one hour. And when the 1.75 million deaths became attributed directly to Khan is equally murky. Even more difficult to understand is how the idea made it on so many lists of amazing statistics. Regardless, a great many people died at the hands of Genghis Khan or his men. But in a strange, roundabout way, he put back more than he took. Thanks to his far-flung travels and his appetite for women, a 2003 study found that as many as 16 million people alive today -- or about 0.5 percent of the global population -- are descendants of Khan [source: Zerjal, et al.].