It’s a reference to a very old, now (thankfully) generally in disuse board meme, which itself is a reference to author Bill Bryson noting that humans have made one trip to the deepest part of the ocean (the Marianas Trench), and it lasted for a mere 20 minutes, and took place way back in 1960.
I think someone accidentally triple posted the original reference back in the days when the boards were even more temperamental than they are now, and somehow it caught on.
So that’s what any reference to “Once, in 1960, for twenty minutes” means, at least as far as I’ve been able to ascertain.
By the way, there are jellyfish that are virtually harmless. I swam through clouds of what I think we called “comb jellies” or ctenophores, I believe, several years ago in the Keys. They looked much like transparent little (2" - 5") bell peppers. Spooky, but no harm.