Scientists believe they've cracked one of the oldest mysteries of a disappearing civilization. The culprit? Climate change. Nearly 5,000 years ago the Harappan or Indus people created a society on the fertile Indus Valley that is now India, Pakistan, Nepal, and Bangladesh—rivaling contemporary civilizations like Mesopotamia and Egypt, and making up 10 percent of the world's population. But nearly 2,000 years later its cities that spanned over 380,000 square miles were abandoned and crumbling.