The second single from Marvin Gaye's landmark What's Going On album, "Mercy Mercy Me (The Ecology)," is one of the greatest, most poignant environmental anthems ever recorded. The song hit No. 4 on the Billboard Pop Chart and No. 1 on the R&B Chart. More importantly, it shared an important message. In three minutes and 14 seconds, Gaye confronted air pollution, poisoned seas, radiation leaks and over-population in a way that's just as true — if not more so — some 45 years later. Buy it here. —Aaron Brophy