Had Zeppelin broken up in 1970 after just two albums, they'd still go down in history as articulating a whole new heaviness. But that was only the beginning, as the Brits churned out a slew of innovative recordings by shading their bluesy wallop with psychedelia, folk, funk, and even reggae. Robert Plant's banshee wail and John Bonham's leviathan grooves reach peak rock heft on the rippling “Whole Lotta Love.” Yet that same band could unplug and deliver “Going to California”—cosmic balladry at its most fragile and glowing.