The Land That Time Forgot is a science fiction novel, the first of Edgar Rice Burroughs' Caspak trilogy. His working title for the story was "The Lost U-Boat." Starting out as a harrowing wartime sea adventure, the story ultimately develops into that of a fantastical lost world. (Adapted from Wikipedia.)
Link to The People that Time Forgot (Caspak series #2)
Link to Out of Time’s Abyss (Caspak series #3)