Harappan, or Indus civilization, arose in Pakistan and northwestern India around 2,500 B.C. This is a time when the great cities of Mohenjo-daro and Harrappa were urban centers. They were artists, builders, and writers. Archaeologists first discovered this civilization in the early 20th century when square stamp seals were found on the banks of the Ravi River in Pakistan. The writing on the seals was unknown at the time, so in 1920 Sir John Marshall and Rai Bahadur Daya Ram Sahni began excavations.