When 19th century explorers and 20th century archaeologists rediscovered the ancient Indus Valley civilization, the history of the Indian sub-continent had to be rewritten.* Many questions remain unanswered.
The Indus Valley civilization is an ancient one, on the same order as Mesopotamia, Egypt, or China. All these areas relied on important rivers: Egypt relying on the annually flooding Nile, China on the Yellow River, the ancient Indus Valley civilization (aka Harappan, Indus-Sarasvati, or Sarasvati) on the Sarasvati and Indus rivers, and Mesopotamia outlined by the Tigris and Euphrates rivers.
Like the people of Mesopotamia, Egypt, and China, the people of the Indus civilization were culturally rich and share a claim to the earliest writing. However, there is a problem with the Indus Valley that doesn't exist in such pronounced form elsewhere.
Evidence is missing elsewhere, through the accidental depredations of time and catastrophes or deliberate suppression by human authorities, but to my knowledge, the Indus Valley is unique among major ancient civilizations in having a major river disappear. In place of the Sarasvati is the much smaller Ghaggar stream that ends in the Thar desert. The great Sarasvati once flowed into the Arabian Sea, until it dried up in about 1900 B.C. when the Yamuna changed course and instead flowed into the Ganges. This may correspond with the late period of the Indus Valley civilizations.
* Mohenjo-Daro - From Archaeology at About.com
The mid-second millennium is when the Aryans (Indo-Iranians) may have invaded and possibly conquered the Harappans, according to a very controversial theory. Before then, the great Bronze Age Indus Valley civilization flourished in an area greater than one million square km. It covered "parts of Punjab, Haryana, Sindh, Baluchistan, Gujarat and fringes of Uttar Pradesh"+. On the basis of artifacts of trade, it appears to have flourished at the same time as the Akkadian civilization in Mesopotamia....