Finding this in a Rolling Stone from awhile ago…

2. ‘Nightswimming’
This song began with a very simple piano melody by the band’s bassist Mike Mills. He didn’t see much value in it, but Michael Stipe thought it was brilliant and he wrote this tender song about skinny-dipping around it. The string arrangement was written by another famous bassist: Led Zeppelin’s John Paul Jones. The track didn’t do that well as a single, but in the past 20 years it’s slowly become one of R.E.M.’s most beloved songs.

R.E.M. - Nightswimming