And they were clean. Most homes were connected to a centralized drainage system that used gravity to carry waste and water out of the city in big sewer ditches that ran under the main avenues, a plumbing system that would have been the envy of many 18th century European cities. Also, in Mohenjo-Daro, the largest public building was not a temple or a palace, but a public bath, which historians call the Great Bath. We don't know what the Great Bath was used for, but since later Indian culture placed a huge emphasis on ritual purity, which is the basis for the caste system, some historians have speculated that the bath might have been like a giant baptismal pool. Also, they traded. One of the coolest things that the Indus Valley civilization produced were seals, used as identification markers on goods and clay tablets. These seals contain the writing that we still can't decipher and a number of fantastic designs, many featuring animals and monsters.