* The civilisation was very ancient. It flourished from about 2500 BC to 1900 BC, which means that it is over 4,000 years old. This makes it difficult to find evidence about the civilisation, as many of the records and artefacts from that time have been lost or destroyed.
* The civilisation was located in a remote area. The Indus River Valley is located in what is now Pakistan and India, which was not a major centre of civilisation at the time. This meant that the civilisation was not well-known to other cultures, and there are few records of it from outside sources.
* The civilisation was not very literate. The Indus Valley people did not have a written language, which makes it difficult to learn about their history and culture.
* The civilisation was destroyed by natural disasters. The Indus River Valley was prone to flooding, and it is believed that the civilisation was destroyed by a series of floods around 1900 BC. This destroyed many of the records and artefacts from the civilisation, making it even more difficult to learn about it.
Despite these challenges, historians have been able to learn a great deal about the Indus River Valley civilisation. They have excavated a number of sites, including the cities of Mohenjo-daro and Harappa, and have found a wealth of artefacts, including pottery, tools, and jewellery. These artefacts have helped historians to learn about the civilisation's economy, technology, and culture.
However, there is still much that we do not know about the Indus River Valley civilisation. We do not know what language they spoke, what their religion was, or how they were governed. We also do not know why the civilisation collapsed. These are just some of the mysteries that historians are still working to solve.