Perhaps the first unfathomable mystery in rock 'n' roll was Elvis Presley's "Blue Moon," recorded in 1954 at Memphis's Sun studios, with Sam Phillips producing, and Scotty Moore and Bill Black providing support on guitar and bass. This unassuming little thing almost always requires a twist of the volume knob. It sneaks in like one late to the curtain, and then somehow becomes the show itself. Its clip-clop rhythm, its heavy echo on Presley's keening delivery, and its utter hush remain as strangely affecting as lifting one's head to the night sky and seeing UFOs: a confluence of imagination and technology at once transcendent, powerful, and elusive. Where does it come from? What does it want from us? As vinyl, memory etched as permanent artifact, it is ultimately more dependent on production than performance. But the performance is not without its points, as when Elvis glides out like some cartoon character swirling over the edge of a cliff, and floats there, buoyed by his own tremulous incoherent falsetto, unencumbered by everything but the sounds of vowels. Its strangeness has long invited interpretations related to death and the afterlife and other mysteries of the universe. It was never a hit of any kind, though it showed up here and there often enough: thrown onto Presley's first album along with other unreleased Sun material; then released as the A-side of a no-hit single in September 1956, a casual afterthought for RCA, busy by then spewing out Elvis product in every direction. It also appeared on an EP that year and, 20 years later, on the essential Sun Sessions album. All who have heard it remember it. Many think it a little weird.