Five Thousand Miles Underground by Roy Rockwood

Dismantling the dirigible-ship Monarch & the submarine Porpoise, our crew constructs from the combined parts the Flying Mermaid which is both airship & submarine. They leave their home in Maine & head for a region of the South Polar sea where they previously found an opening into the Hollow Earth & fly right in. Deep below the earth they discover a lost race of giants & experience a tremendous lot of adventures involving a murderous plant, weird animals, & a subterranean temple of treasure. Using parts taken from the dirigible-ship Monarch and the submarine Dolphin our crew constructs the Flying Mermaid, an airship-submarine! They take their new vehicle to the South Polar sea and discover an entrance in the Hollow Earth, where they find a lost race of giants, murderous plants, weird animals, and a temple full of treasure...