"Mercy, Mercy Me (The Ecology)" (Marvin Gaye)

After proving Motown president Berry Gordy wrong with the huge success of the single "What's Going On" (Gordy had initially refused to release the song, feeling it was uncommercial and would alienate longtime fans with its social message), Marvin Gaye continued in his social consciousness vein with "Mercy, Mercy Me (The Ecology)."  
After taking an extended break from recording, largely due to being depressed about the death of his friend and singing partner Tammi Terrell, who'd passed in 1970 from a brain tumor, Gaye decided that, if he was going to record again, he only was going to make music that spoke to things he cared about: love, life, God, people, and nature.
As Gaye told Britain's Disc and Music Echo in 1971, "I can remember as a child I always kept myself to myself, and I always dug nature.  I used to fool around with worms, beetles, and birds, and I used to admire them while the other kids were playing sports.  It was like some strange force made me more aware of nature."
It probably seemed completely out of left field at the time for an R&B star to release a song about environmental concerns, but Gaye felt it was his duty to say something.
Incidentally, Gordy didn't care for "Mercy, Mercy Me" either.  (Apparently, he also had to be told what ecology was.)  But, once again, he changed his tune when the single version became a Top 10 seller in 1971.
Apart from Gaye's heartfelt delivery and poignant commentary about the ills mankind had inflicted upon the environment, I love the sound of this track.  There's the deep, signature Funk Brothers groove and Wild Bill Moore's unhinged sax solo, but then there's also that echo created by the heavy reverb on the percussion instruments.  That spacious sound created by the woodblock and congas is so evocative of the song's somber mood; it not only sets the scene that Gaye is drawing upon distant memories of better times, but it also replicates the paved-over, hollow sound of inner cities in that era.
In fact, if you are old enough to remember what most downtowns across America looked like in the 70s to the mid-80s, you'll know what I mean: there often were few trees, lots of concrete and asphalt, and lots of abandoned buildings.  In that emptiness, the cold sounds of vehicles and machines were amplified, including the din of demolition—the crashing and banging of falling debris and dump trucks.  Those were the sounds I heard as a child when my parents would take me to downtown Asheville.  Or when we visited relatives in the Rust Belt.  Or when we went to D.C. for the first time and stayed on South Capitol Street.
And it was that sound that struck me the first time I heard the song on the radio as a kid.  I remember being amazed that a sound could capture a time and place so precisely.
And I still marvel at Gaye's brilliance to not only use his words but also the very aural texture of America's crumbling cities to reach listeners.  It's simply a great and important song.