The head of Buddha trapped amidst the roots of a banyan tree at Wat Mahathat in the ancient ruins of Ayutthaya is probably the most iconic image associated with Ayutthaya, the former capital of Thailand. If you do a Google image search for Ayutthaya Thailand, “the head of Buddha overgrown by the roots of a banyan tree” will appear several times on the first page.
Exactly how the head of Buddha came to rest amidst the roots of this banyan tree is a mystery, but Ayutthaya was over run by the Burmese in 1767, much devastation was inflicted upon Ayutthaya and the temples were not spared.
The head of the Buddha probably came to rest on the ground during the conflict, the banyan tree which has grown nearby has aerial roots which have grown downwards into the ground and in the process have grown around the Buddha head. That’s one theory anyway, got one of your own, just drop it in the comments.