Abraham Merritt 
The Moon Pool [EPUB ebook] 

First published in 1919, ‘The Moon Pool’ embodies all the romanticism and poetic nostalgia characteristic of Abraham Merritt’s writings and has become one of the most gripping fantasies ever written.
Set on the island of Ponape, full of ruins from ancient civilizations, the novel chronicles the adventures of a party of explorers who discover a previously unknown underground world full of strange peoples and super-scientific wonders. From the depths of this world, the party unwittingly unleashes the Dweller, a monstrous terror that threatens the islands of the South Pacific. 
The protagonist, Dr. Goodwin, would later appear in Merritt’s second novel ‘The Metal Monster’, originally published in 1920.
Although Merritt did not invent the lost world novel, following in the footsteps of Sir Arthur Conan Doyle, Burroughs and others, he greatly elaborated upon that tradition.