The Mermaid of Maracot Deep

The beautiful and happy-looking mermaid on the cover caused me to do this conversion from an Internet Archive magazine. Who knew that she was a monster, whose diet was raw humans?

 The story, unfortunately, is terrible science fiction, about the Bikini atomic test opening a hole in the earth's crust. An inner world is revealed with giant mer-people, bubble-cities of humans, impossible gravitational anomalies, and so on. An of course, in the end, the Captain gets the girl!