The deserts of Saudi Arabia are still holding on to many ancient secrets, hidden inside burial tombs and mysterious monumental structures called mustatils. Dr Hugh Thomas is on an archaeological mission to solve some of these mysteries.Hugh Thomas is an archaeologist who is fascinated by ancient mortuary practices and the secrets still hidden in the deserts of Saudi Arabia.In the north west of the country, thousands of mysterious rectangular structures, built in the fifth millennium, are still standing.They are monumental structures, up to 600m long, built from walls of rock and best viewed from the sky, where the chambers in which ritualistic killings took place, are clear. But who or what exactly motivated these ancient architects to build such things is not yet clear.And crisscrossing the landscape around them are kilometres of pathways called 'funerary avenues' -- routes carved out by people and herds, punctuated by burial tombs that look like jewellery from the air.This episode of Conversations explores ancient history, deep time, epic discoveries, the Middle East, Saudi Arabia, death and archaeology.