Around 4,000 years ago, one of the world's oldest civilizations emerged: The Indus Valley Civilization, flourishing in what is now Pakistan, western India, eastern Iran and parts of Afghanistan. In addition to building sizable cities, its people created a written script that consists of hundreds of signs that remain undeciphered.
The signs, sometimes called Harappan script, vary, with some looking like a diamond with a square in its corner; a U with three "fingers" at each end, and an oval with an asterisk-like shape inside it.
