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 Iraqi, British and Dutch team of archeologists has found evidence to confirm the theory. For the first time, the team dated some of the massive earthen structures and found that their construction 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.