A system of thousands of ridges and canals across a floodplain in southern Iraq has long been believed to be the remnant of a massive agricultural system built by slave labor. Now an international team of archaeologists has found new evidence to support the theory. The team undertook testing to determine the construction dates of some of the massive earthen structures and found that they spanned several centuries, beginning around the time of a famous slave rebellion in the 9th century A.D. The research findings were published Monday in the journal Antiquity.