Perhaps it's safe to say that The Time Tunnel was ahead of its time.
Irwin Allen's 3rd series, this one for ABC, lasted just one season, but the man-out-of-time concept has been revised and repeated time and again in other series in years since, including the animated Happy Days spinoff, Fonz & The Happy Days Gang and the live-action Voyagers!, both in the early 80's.
Two scientists (Robert Colbert & James Darren, to reverse the billing) are involved in a time travel experiment that goes wrong, sending them into historical events and places. Voyagers! would use this same concept more than 15 years later, and 20 years after that, the animated Time Squad crossed Voyagers! with Peabody's Improbable History, but with minimal success. The common thread is that except for Fonz, none of these shows lasted more than a season, really, though back in the 60's, the standard primetime order was in the vicinity of 30+ episodes, which has since been trimmed in intervening years to 22-24.
James Darren was already well known as both an actor and a pop singer, with his most notable hit being "Goodbye Cruel World", and had guest-starred on The Flintstones, as well as working on another Hanna-Barbera project, the feature film Hey There, It's Yogi Bear! (and, yeah, he sang in that one, too). Robert Colbert was the lesser known star, and drifted back into obscurity after Tunnel ended. Worth noting, too, is that the cast also included Lee Meriweather, who subbed for Julie Newmar as Catwoman in the Batman movie spinoff earlier that summer, and would later land a more successful gig in Barnaby Jones, opposite Buddy Ebsen.
Dinadangdong uploaded the open, with the theme composed by future Oscar winner John Williams.
Clearly, Allen's magic was running out, as the series that replaced Tunnel, Land of the Giants, would arrive the next year. We'll take a look at that another time.