Somewhere in San Diego a tantalizing Holy Grail of rock performance history is awaiting discovery in a dusty corner…maybe.

As documented in Richie Unterberger’s comprehensive new book White Light/White Heat: The Velvet Underground Day by Day, Lou Reed’s influential avant rock ’n’ roll band played San Diego twice. Both were multiple-night engagements at the Hippodrome, a long-gone converted roller rink at Front and G streets. The Velvets played there in mid-June 1968 (favorably reviewed in the San Diego Union) and returned in early July for dates with San Francisco’s Quicksilver Messenger Service.

An impressed Lester Bangs of El Cajon, 19, was there in July; later, as a top rock critic, his championing of VU and Reed helped enshrine them in the canon of cool. “In a way it was the ultimate Velvet Underground concert,” he wrote in a 1971 article for Creem. “The audience was terrible…they wanted those California acid vibes instead of what they took for cold New York negativism.…

“Right in the middle of all these bad vibes, the Velvets launched into a new song that was one of the most incredible musical experiences of my concert career.… It was the chorus that was the most moving: ‘Ohhhh, sweet rock and roll — it’ll cleanse your soul.’ That’s classic, and no other group in America could have (or would have) written and sung those words.”

As late VU guitarist Sterling Morrison told Spanish journalist Ignacio Julia in 1986, at a post-gig party, he and Reed listened to a four-track SD show recording: “Then came ‘Sweet Rock and Roll,’ which was never recorded [in studio] and never played again. [That] tape…sounded so great…there was no point in recording it.” But as Morrison elaborated in an unpublished interview with M.C. Kostek and Phil Milstein of What Goes On magazine, “The tape was stolen that very night.… [I]t vanished, never to reappear on earth.”

“If the tape has good sound, certainly it would be a Holy Grail for VU fans,” agreed Unterberger by email. The missing recording is on his book’s “Ten Best Unreleased” list. “There aren’t any high-fidelity live tapes of the [John] Cale lineup in 1968, and as this is just a few months before he left the group, it probably shows that lineup approaching their live peak. It would be added value if ‘Sweet Rock and Roll’ is just as exciting as Bangs and Morrison claimed it to be.… If one of [their recently discovered] 1966 acetates got $25,000 on eBay, I’d say such a tape should be worth something in that neighborhood.”