The Indus Valley Civilization (IVC), also known as Harappan Civilization flourished around 2,500 BC in the western part of South Asia. The Harappan culture was distinguished by its system of town planning.

Significant features of the Harappan town planning are as follows:

• Division of city into the Citadel e. mound built on the high podium and the lower town containing brick houses inhabited by the common people.
  
  
• The arrangement of the houses in the cities followed a grid system.
  
  
• Advanced drainage and sanitation system of each house were connected with road drains that were covered by stone slabs.
  
  
• The other significant features were well-arranged water supply system, street lightning system, designated places to throw waste material
  
  

The significant features also show high level of sophistication in town planning as: 

• The use of burnt bricks in the Harappan cities was remarkable as buildings of Egypt mainly used dried bricks.
  
  
• Harappans laid special emphasis on health and hyenine as seen from bathroom in every house and well laid drainage system.
  
  
• The town planning also kept into mind the need for storehouses for having provision during emergency. For Ex.- Great granaries of Harappa.
  
  

However, the Harappan civilization began to decline around 1800-1500 BC and some of the reason/ theories proposed by historians are as follows:

• The massive floods in the Indus may have been a potent cause for the extinction of the Harappan culture.
  
  
• Repeated seismographic vibrations must have also led to erosion of decline of Harappan civilization as it lied in high seismic region.
  
  
• Further, water scarcity must have led to the exodus of the Harappan people to other places.
  
  
• Outbreak of the plague epidemic is also shown as a reason for the decline of Harappan civilization.
  
  
• Prof D.D. Kosambi is of the opinion that the Aryan invasion is the reason for the decline of Harappan culture.
  
  

Thus, the multiple causes/ factors have been proposed to be responsible for the decline of Harappan culture.