The 30 Years Cave is a cave found only in Pokémon Brown, located in the southern section of Route 52 and just north of Seashore City. According to a book in Koolboyman Mansion, it is a strange cave that only appears every 30 years, hence the given name. However, the cave appears to have no in-game name, with none listed on its Town Map entry.
In the top center of the cave is a level 50 Entei or Moltres. In order to reach this section, a boulder-pushing puzzle must be completed. The ladder in the very lower right of the cave will then be accessible, which leads to the central area with the legendary pokémon encounter.
This cave also appears in Pokémon Prism, but its entrance is sealed.