Complex sentence: The tomb is thought to have been built at some time between the end of the 7th century and the beginning of the 8th century, but was accidentally discovered by a local farmer in the 1960s.
Split into simpler sentences: The tomb is thought to have been built at some time between the end of the 7th century and the beginning of the 8th century. It was accidentally discovered by a local farmer in the 1960s.