I am just telling you about harappan civilization. you believe or not it depends upon you.
According to ancient history harappa was a very well developed civilization. And no one knows from where such type of technology had came. It has well developed housing,sewage system just like we have this time in developed areas.According to me such technology had came or brought by aliens which may came earlier and they tell only a very little about their technology but this little technology is so much in front of us.This shows that we are very less developed on comparing with ancient aliens.
Watch ancient aliens if you are interested in the aliens