Classics are books that remain timeless no matter in which timeline they take place originally, whether it's in ancient Greek, Roman myths, or back in the 1800s. They can be books with leather-bound, novels that are instructed to be read by kids in schools, or simply stories that explore the world in a way we've never thought about before because they were visionaries.
Don't get me wrong, classics aren't underrated, unlike other books included on this website. But some of them were forgotten one way or another. There are people that only think about the movie adaptation of The Great Gatsby, with no thoughts for the original material. Classics, by definition, are popular, but they can't all escape the possibility of oblivion. It is our role, as readers, to keep them in our memories and bookshelves.