A serious and clearly troubled Fallon introduced his hilarious bit with a somber face and downbeat voice.
"Earth day marks the 5 years since the BP oil spill," he told his Australian guest. "BP has left tar balls all over the sea," he continued. "Don't go swimming down in the south, unless you want tar balls in your mouth."
Getting angry about the fact that BP didn't seem to have gotten his message, Fallon then gave the Noah actor, 51, a little taster of his song.
Clearly roused by the music, Crowe declared himself to be feeling the message very deeply, and grabbed a guitar to join Fallon for a very memorable duet!
Watch the hilarious video above and see what happens when the musical pair take their song into the audience!