|They don't know, therefore...|
|Xenu was here|
Mohenjo-daro is an archeological site of one of the largest Indus Valley civilizations that thrived around 2000 BCE. Pseudohistorians and proponents of ancient advanced civilizations or extraterrestrial visitations claim that there is evidence that its population was killed off by an atomic bomb explosion around 1500 BCE. The claim is ridiculous and the proffered evidence is rife with pseudoscience; this, of course, didn't stop it from being featured prominently on the History Channel's show Ancient Aliens.
 Mud brick buildings
The biggest problem with the nuclear blast claim at Mohenjo-daro is actually just looking at pictures of the dig site. The buildings were made with kiln fired mud bricks. Some of the walls are still 15 feet high. How a mud brick wall withstands a nuclear blast never seems to be addressed.
The primary evidence cited as proof of an atomic bomb being used on Mohenjo-daro is the discovery of a series of bodies throughout the cities. It is presented as a single instantaneous destruction of the population of the city:
“”when excavations of Harappa and Mohenjo-Daro reached the street level, they discovered skeletons scattered about the cities, many holding hands and sprawling in the streets as if some instant, horrible doom had killed its inhabitants. People were just lying, unburied, in the streets of the city; there seemed no-one available to bury them afterwards.
As with all pseudohistory and pseudoscientific tirades the actual evidence is far different, more subtle and more interesting. The bodies were discovered during excavations between 1922-1931. Scholars have criticized the methodology used during the dig period and have stated that making any sort of interpretation is difficult. Even though some of the original excavators who examined the evidence and saw the bodies as linked to a single catastrophic event but their opinion was that it was more akin to war and not nuclear explosion. 
Thirty seven bodies total were found, , some of them were found in the same parts of the city but based on distribution in the stratum in relation to the buildings different groups of bodies came from different time periods [source needed]. Mohenjo-daro had three major periods, early, intermediate and late. Some groups of bodies were clearly early period and some clearly late, a period of a thousand or more years [source needed]. Another key point is that the bodies actually showed clear signs of burial [source needed]. The most infamous group of bodies, as highlighted from the quote above, that were found laying in the middle of the street was actually caused by the fact that the bodies were buried during the later period when the existing buildings had been built over earlier periods. The bodies were buried above a road from a period hundreds of years earlier.
Proponents of the bomb explosion often like to point to the preservation of the bodies, particularly the lack of signs of scavengers as proof that something like a background radiation had to keep animals away. Several important points about this, the first is this is one of the driest and hottest spots on the planet, perfect weather for preservation. The scavenger problem is only a problem for the pseudohistorians who seem unaware that the bodies were actually buried.
Small groups of bodies dating from multiple periods a thousand years apart, with clear signs of burial, and some stratum coincidences, clearly is weak evidence for an atomic explosion. The evidence is also nothing like what is presented on proponent websites or television. An interesting fact though is that this is not the first time that the Mohenjo-daro bodies were the subject of controversy. The original excavators tried to use them as proof that the indo valley civilization was killed by invading hordes of Aryans. Much of the evidence presented showing that the bodies were not due to a single large scale event was elucidated 50 years ago to disprove the invading horde hypothesis.
Beyond the bodies themselves pseudohistorians make a series of claims about radioactivity levels in the area. These claims range from a radioactive dust covering the whole city, to the bodies themselves being "among the most radioactive ever found, on par with those at Hiroshima and Nagasaki." The problem with these claims is that they are so vaguely sourced its impossible to find the original claims to verify it. Other radioactivity based claims are easier to refute.
One common claim is that the sand around the city had vitrified similar to the trinity site in New Mexico. However, what was actually found was the dumping ground for broken pots that were often made by vitrifying sand in a kiln.
 What really happened
The ancient cities weren't built near any source of water like river or lake. The Indus River near which Mohenjo-daro was built, gradually changed its path so the residents were left with no source of water. Hence, they gave up the city and moved to some other place near to water. Its been speculated that the residents of Mohenjo-daro, after leaving the city got settled in the banks of river Ganges.