The Volcano is an area where Pokémon can spawn. It is between the 4th and 5th Gyms.
To get thereEdit
Facing the 4th gym, it is the doorway up high to the right. It requires the Sun badge to enter.
In the volcanoEdit
There are 2 doors. One is to the 4th gym, the other on the cliff in to the 5th gym. There is a path leading across the Volcano ending at the doorway to the 5th gym. Around the path is lava. You may walk on the lava and jump back onto the path where it is low enough. Pokémon spawn on the lava, near the doorway to the 4th gym and on an island connected to the path.