"The Land That Time Forgot" is a 1918 science-fiction novel by Edgar Rice Burroughs. It was originally published in a serialized form, which probably accounts for its episodic structure.
The story follows one Bowen Tyler, a former submarine builder who is on a freighter in the North Atlantic which is coincidentally downed by a German U-Boat that he himself constructed. He and the crew of the freighter manage to take over the submarine that sunk them, only to find out that their lone female survive, who our protagonist has already taken a shine to, was at one point engaged to the submarine's commander, a stern Prussian noble. (Now that I type this out, this is a lot of coincidences, isn't it?). The submarine sails around the world with the Germans and English taking turns hijacking it from each other.
All of this is a prelude to what the book is really about, which is what the title says: The Land that Time Forgot. After drifting into the Pacific, the crew find a large island that is apparently the caldera of a gigantic, extinct volcano. They use the submarine to swim through an underground cave and find themselves in a lush, violent world full of dinosaurs and other prehistoric creatures, as well as early humanoids. Survival in this wild land, as well as the continuing problems with the captured German prisoners, makes up the rest of the narrative.
This book is prototypical pulp. There is a lot of flaws in this book: the disjointed structure, the unbelievable coincidences, the racist caricatures, the suspect science, the one-dimensional characters. But if you can put aside critical thinking for a while, and enjoy this book as a formative work of science fiction, it still is entertaining, and important in the development of science fiction. Just don't ask too many questions.