The ruins of two ancient cities, Harappa and Mohenjo-Daro (both in modern-day Pakistan), and the remnants of many other settlements, have revealed great clues to this mystery. Harappa was, in fact, such a rich discovery that the Indus Valley Civilization is also called the Harappan civilization.

The first artifact uncovered in Harappa was a unique stone seal carved with a unicorn and an inscription. Similar seals with different animal symbols and writings have since been found throughout the region. Although the writing has not yet been deciphered, the evidence suggests they belonged to the same language system. Apparently, Mesopotamia's cuneiform system had some competition in the race for the world's first script.

The discovery of the seals prompted archaeologists to dig further. Amazing urban architecture was soon uncovered across the valley and into the western plains. The findings clearly show that Harappan societies were well organized and very sanitary.

For protection from seasonal floods and polluted waters, the settlements were built on giant platforms and elevated grounds. Upon these foundations, networks of streets were laid out in neat patterns of straight lines and right angles. The buildings along the roads were all constructed of bricks that were uniform in size.

The brick houses of all city dwellers were equipped with bathing areas supplied with water from neighborhood wells. Sophisticated drainage systems throughout the city carried dirty water and sewage outside of living spaces. Even the smallest houses on the edges of the towns were connected to the systems — cleanliness was obviously of utmost importance.

These Ancient drains are in a side street in the city of Mohenjo Daro. The drain in the center street is joined with smaller drains from each house. photo from The Indus Valley Civilization - Weebly https://theindusvalleycivilization.weebly.com/uploads/2/0/5/0/20503746/3985906.jpg?350