The town of Hannibal was immortalized as "St. Petersburg" by Mark Twain in The Adventures of Tom Sawyer. This included the notorious McDougal's Cave, renamed Mark Twain Cave and turned into a commercial cave in 1886. The owner discovered another cave in 1925, now called Cameron Cave, that is minimally developed. Both caves are fragments of a larger complex that was dissected by erosion. They are unusual, especially among Missouri caves, in being nearly free of speleothems (stalactites and so on). Instead they formed as networks of tall, narrow passages, opened by groundwater, that follow the major joints in the rock. This accounts for their labyrinthine character and relatively dry condition.