6. The civilization that laid the bricks, one of the world's oldest, was known as the Indus. They had a written language and were highly sophisticated. Dating back to 3000 BC, they originated in the south and moved north, building complex, mathematically-planned cities. Some of these towns were almost three miles in diameter and contained as many as 30,000 residents. These ancient municipalities had granaries, citadels, and even household toilets. In Mohenjodaro, a mile-long canal connected the city to the sea, and trading ships sailed as far as Mesopotamia. At its height, the Indus civilization extended over half a million square miles across the Indus river valley, and though it existed at the same time as the ancient civilizations of Egypt and Sumer, it far outlasted them.
