Mary Shelley states in the introduction that in 1818 she discovered, in the Sibyl's cave near Naples, a collection of prophetic writings painted on leaves by the Cumaean Sibyl. She has edited these writings into the current narrative, the first-person narrative of a man living at the end of the 21st century. (Excerpt from Wikipedia)
Mary Wollstonecraft Shelley (née Godwin; 30 August 1797 – 1 February 1851) was an English novelist, short story writer, dramatist, essayist, biographer, and travel writer, best known for her Gothic novel Frankenstein: or, The Modern Prometheus (1818). (Wikipedia)