From 1936, this is the one where they find Atlantis (here called TAZ).

Pretty good story, better than I had remembered -- think I was
confusing it with several other Caribbean ocean adventures.

Different groups of competing bad guys (I had forgotten the flamboyant
Captain Flamingo, who dresses in all different colors).

What got my attention was this -- almost all of the story is set-up.
Running around NYC, hunting down clues, actually getting to TAZ. Then
it wraps up in twenty pages. From the point of view of story
structure, this would have been a natural two-parter, continued story.
Easy to pad the word count to expand the first part, then tease with
the reveal to have readers just waiting for the second part.

Why didn't the hero pulps go with contuned stories. Serials were a
constant in all other genres. We get recurring bad guys in the hero
pulps; OPERATOR FIVE and DUSTY AYERS had long continuing plotsl why
never a TO BE CONTINUED?

Dave