"Niels Klim's journey under the ground" by Ludvig Holberg is a satirical science-fiction novel published in 1741. When a curious philosophy student investigates a mysterious cave in Bergen, he falls through the earth and discovers an inner world orbiting a subterranean sun. There he encounters walking, talking trees, explores utopian societies with radically different values, and faces exile for challenging their customs. His twelve-year journey through this underground realm becomes a biting commentary on religion, government, equality, and human nature. (This is an automatically generated summary.)
These books are free to read. Each is in the public domain in the United States; check your own country's rules before you copy or share one.