On various sculptures of Sanchi Stupa, there is a carving of stupa. What does this stupa signifies as per the historians?

Mahaparinibbana

According to hagiographies, the Buddha attained enlightenment while meditating under a tree. Many early sculptors did not show the Buddha in human form – instead, they showed his presence through symbols. The empty seat was meant to indicate the meditation of the Buddha, and the stupa was meant to represent the mahaparinibbana. Another frequently used symbol was the wheel. This stood for the first sermon of the Buddha, delivered at Sarnath. As is obvious, such sculptures cannot be understood literally – for instance, the tree does not stand simply for a tree, but symbolises an event in the life of the Buddha.