Proof positive that the Beatles were evolving at lightspeed arrived in 1964 with the release of the LSD-spiked Revolver, a confidently - if tunefully - experimental LP bursting with studio trickery, unusual instrumentation and equal measures of whimsy and menace. For every sun-dappled jaunt like "Yellow Submarine" or "Good Day Sunshine," there was a shadowy counterpart like the gnarly "Doctor Robert" or "Tomorrow Never Knows," which featured John Lennon intoning portions of the Tibetan Book of the Dead over a squall of tape loops and mutilated guitars. A wild ride to this day.

First listen: