What do you get if you cross a Sasquatch with a Tree Octopus? Perhaps something like this:
Synopsis: A submarine suffers some type of accident (again, missing pages) in the Pacific and emerges in a strange world inhabited by "octopus-men" -- a kind of hominoid/octopus version of a centaur. The two surviving submariners, Harvey and Taylor, are captured by these octopus-men and taken, along with the corpses of their dead shipmates, through a strange, rubbery forest to the octopus-men's cavernous lair.
Harvey theorizes to Taylor that they fell through a hole in the bottom of the sea (a hole in the bottom of the sea) and are under the seafloor, Hollow-Earth-style. As they're carried by their heads into the lair, they see chambers filled with hundreds of years worth of skeletal remains from sailors, which Harvey suspects were sucked down here after drowning. Their captors seem both fascinated and annoyed to see living humans.
The octopus-men glue the two humans to a cave wall using their saliva, which they also use to weave a web blocking the cave entrance, then leave the men to their fate. The cave is a hatchery and is soon seething with larval octopus-men. The larvae swarm over Harvey and Taylor and start gnawing at them, breaking the saliva-bonds. This drives Harvey to mad laughter, which scares the larvae, causing them to flee the men by gnawing open the doorway. Escape now possible, Taylor has to knock out Harvey, who is helplessly hysterical, and carry him out of the cave to freedom.
When Harvey comes to, they are now on the shore of a normal, octopus-men-free island. Taylor tells him that when they left the cave everything dissolved, then they were here. Turns out it wasn't the Hollow Earth after all but the Fourth Dimension "impinging" on the island. At least that's what Taylor now theorizes, not that he's one of them bookworms or anything, but that's what them modern scientist types keep saying, so seems more likely than a hole in the bottom of the sea. (The titular "space monsters" weren't from outer space or inner space, but from parallel space.)
There was a tidal wave and their submarine was washed inland. The skeletons they saw were other castaways on the island from previous dimensional impingements. They're still in danger! The octopus-men-filled fourth dimension could come back at any time! Fortunately a ship just happens to sail by at that moment and rescues them. The ship captain tells them it's a bad island, then they all leave quickly. The End.
An abrupt and pointless denouement, but still more satisfying than Lost.