Mohenjo Daro likely was, at its time, the greatest city in the world. Roughly 4,500 years ago, as many as 35,000 people lived and worked in the massive city, which occupies 250 acres along PakistanÕs Indus river. Mohenjo Daro sat beneath the soil for thousands of years, a preserved relic of the ancient Indus Valley civilization. But excavation exposed the city to the elements, and now, says the Telegraph, the ruins may have as little as 20 years left.