The True History (Greek: Ἀληθῆ διηγήματα, Alēthē diēgēmata; Latin: Vera Historia) by Lucian (c.115-c80 CE) is a parody of traveller's tales which features journeys to the moon, and to the Isles of the Blest. It influenced a later genre of fantasic voyages to which works by authors such as Rabelais and Swift could be ascribed, and has been called the first work of science fiction.
Free online texts
The Classics pages: Lucian - Greek science fiction. HTML format.
Gutenberg: Lucian's True History, translated by Francis Hickes. Multiple formats.
Loebulus: L014 - Lucian -- Lucian I: Phalaris. Hippias or The Bath. Dionysus. Heracles. Amber or The Swans. The Fly. Nigrinus. Demonax. The Hall. My Native Land. Octogenarians. A True Story. Slander. The Consonants at Law. The Carousal (Symposium) or The Lapiths. Public domain Loeb edition, Greek-English parallel text. PDF format.
The Lucian of Samosata Project: The True History, translated H.W. Fowler and F.G. Fowler. HTML format.
Perseus: Greek text. HTML and XML formats.
Sacred Texts: A True Story, translated by A.M. Harmon (1913). Parallel Greek/English HTML text.
University of Adelaide (Internet Archive): The True History, translated by H.W. Fowler and F.G. Fowler. Multiple formats.
The Encyclopedia of Science Fiction: Lucian.
Librivox: Instructions for Writing History and True History, translated by Thomas Franklin, public domain audiobook.
The Public Domain Review: Lucian's Trip to the Moon, by Aaron Parrett.
Smithsonian: The Intergalactic Battle of Ancient Rome, by Lorraine Boissoneault, 14 December 2016.
The Telegraph: Is this the first ever sci fi novel, by Charlotte Runcie, 7 November 2013.
Wikipedia: True History.
YouTube: Lucian of Samosata and the True Story, introduction from Professor Tim Whatmarsh, by Tom Mackenzie.
The Great Conversation: Further reading at Tom's Learning Notes
Homer: The Odyssey - Lucian takes Homer's Odysseus as the archetype of the untrustworthy travel narrator.
Herodotus: The Histories - Lucian's travellers meet Herodotus in the underworld where he is punished for his lies.
Rabelais: Gargantua and Pantagruel.
Jonathan Swift: Gulliver's Travels.
Ancient Greek resources: Learn to read Greek classics in the original.