Shoal Cave is found within Pokémon Ruby, Sapphire and Emerald, three games that all primarily take place within the Hoenn region of the Pokémon world. Shoal Cave is special location because the layout of the cave differs depending on the time of day.
The game uses a tide mechanic to make certain areas of the cave available only at certain hours. Between 9 A.M. and 3 P.M., the cave is flooded with water from high tide and it is necessary to use a Pokémon that knows the move Surf to explore the interior. Between 3 P.M. and 9 P.M., the cave is dry and it is possible to explore more floors, including an area with ice. Within the cave is a non-player character who gives out Shell Bells, an item that recovers hit points if held by a Pokémon in battle.