Detective Conan S01 E394 Big Adventure in the Eccentric Mansion: The Seal

"One truth prevails."

Agasa, Haibara, Conan, Ayumi, Mitsuhiko, and Genta go camping. While they're camping they find a lantern in a pond with some writing inside. They show the stone lantern to Conan who determines that it is a code to finding a large diamond. The lantern is also hiding another secret...a dead body at the bottom of the pond.