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In 1957 the archaeologist V. S. Wakankar saw rock formations from a train window near Bhopal, recognised them as resembling painted shelters he had seen in Europe, and went back to investigate. The hundreds of shelters he found hold paintings and occupation deposits spanning from the Palaeolithic to historic times, making Bhimbetka one of the longest continuous records of human habitation in South Asia.
When later images are painted over earlier ones, or later deposits settle on top of earlier layers, the order of overlap gives a relative sequence even when no absolute date is available. A site can be read as a stack of time. Each layer is younger than what lies beneath it, and styles and tools can be ordered by where they sit in the stack.
The Harappan script remains undeciphered, so everything known about the Indus civilisation comes from material remains rather than texts. That uniformity is the evidence for coordinated shared standards, even though no written record states who set them.
Because no later city was built over it, its temples, bazaars, water channels, and royal enclosures survive as a legible layout. Collapse followed by desertion preserved the city better than continued success would have.