What were the sacred relics that were enshrined within stupas?

Fragments of the Buddha's remains

The practice of constructing stupas might have predated Buddhism, yet it became closely linked to the religion. As these structures enshrined sacred relics, they grew to symbolize not only the Buddha but also the essence of Buddhism itself. A Buddhist scripture called the Ashokavadana reports that Asoka divided fragments of the Buddha's relics among significant towns and commissioned the construction of stupas to house them. By the 2nd century BCE, various stupas, such as those in Bharhut, Sanchi, and Sarnath, were already established.