Indus Valley mysteries

Along the Indus River (in today's Pakistan and northwest India), a civilization historians call the Indus Valley or Harappan civilization built some of the ancient world's most advanced cities between about 2600 and 1900 BCE. Its two largest cities, Harappa and Mohenjo-daro, were laid out on a careful grid of straight streets, built almost entirely from standardized, kiln-baked bricks used the same size across hundreds of miles. Even more remarkable, most houses connected to covered drainage channels that carried waste out of the city -- a sewage system more advanced than many cities would have thousands of years later. Mohenjo-daro also contains the Great Bath, a large, carefully waterproofed public structure that may have been used for ritual bathing.

The Indus Valley civilization is also one of history's genuine unsolved mysteries. Its people carved a system of symbols, called the Indus script, onto thousands of small stone seals (often decorated with animals, including a famous design historians nickname the "unicorn seal"), and copies of these seals have turned up as far away as Mesopotamia, proving long-distance trade. But the script has never been deciphered -- no one has found a bilingual inscription to serve as a translation key, so even today no historian can read what it says. Around 1900 to 1700 BCE, these great cities were gradually abandoned. Historians have proposed several theories -- a shifting or drying river system, changing climate and weakened monsoon rains -- but no single explanation has enough evidence to be fully settled, and honest historians say so plainly.