For an album that opens with “Immigrant Song,” arguably the most powerfully condensed expression of their primeval blood-and-thunder heaviness, Led Zeppelin’s third album is best appreciated as an expansion beyond those limitations. This is where they really started figuring out the extra dimensions to their sound, where folk started mingling even more deeply into their proto-metal and their lyrics found further depth and subtlety beyond the first two LPs’ studies in libido. The Plant/Page tandem still has a way with a blues-rock lament, but they’ve started to find new angles; “Since I’ve Been Loving You” filed the serial numbers off Moby Grape’s “Never” and turned it into an epic of slow-burn-to-cataclysm arrangement, eventually to become a concert showcase favorite. And “Out On the Tiles” is one of their best no-bullshit rockers, a dry run for the following year’s “Black Dog” that stands as the LP’s key “holy shit listen to Bonzo’s drumming” highlight. But there’s more atypical and unexpected pleasures in the cuts that classic rock radio forgot: the manic rush of “Celebration Day” and its vertigo-panic Page riffs smuggle in some of their most empathetic, socially perceptive lyrics (“She hears them talk of new ways/To protect the home she lives in/Then she wonders what it’s all about/When they break down the door”) to go with its deceptively giddy chorus, while the Side 2 tandem of “Tangerine” and “That’s the Way” show Plant at his most vulnerable whether he’s worrying about a long-lost love or a world that seems poisoned by alienation.