Trying to remember the name of a book where they travelled to this "perfect" unknown society. They soon realize it's far from perfect and try to leave. The society elders finally let them leave but they wind up 100 years in the past...help!!
I've changed the title for you. How old is this book? It wouldn't be Vingt mille lieues sous les mers: Tour du monde sous-marin (Twenty Thousand Leagues Under the Sea: A Tour of the Underwater World) or the sequel, L'Île mystérieuse (The Mysterious Island)?