http://www.beforeus.com/indusa.htm

Personally, I believe that this was a punishment by God for those who went astray from the true path. God talks a lot about punishing those who go astray, and dread in this life.

Do they not travel through the earth, and see what was the End of those before them (who did evil)? Allah brought utter destruction on them, and similar (fates await) those who reject Allah. - 47:10, Qu'ran.

And listen to this next verse; We know that the Indus Civilization was one of the greatest in the early days.

Do they not travel through the earth and see what was the End of those before them? They were even superior to them in strength, and in the traces (they have left) in the land: but Allah did call them to account for their sins, and none had they to defend them against Allah. 40:21, Qu'ran.