On this encore edition of The 21st: Why it's so tough to keep the world's largest freshwater system clean. Plus, an interview with the mayor of Metropolis, Illinois.

Not too keen to join a combat unit, John Metzger of Metropolis, Illinois jumped at the opportunity to become a chaplain’s assistant after enlisting in the Army in 1969 and being sent to Quy Nhơn, Vietnam. After serving his time, he graduated from Southern Illinois University with a Ph.D. in history.