Secret Base
A secret base is a special area in Generation III and Generation IV where players can hide out. Between the two generations, most of the similarities found are in the way that they can be decorated.
Generation III
Spread across the Hoenn region are caves, trees, and bushes which are perfect to hold a secret base for any Pokémon trainer. These locations can be opened when a Pokémon uses the move Secret Power at: a wall with a small indent; a tree with vines; or where a clump of grass may be moved.
A trainer may decorate their secret base with big and small dolls, mats, desks, chairs, plants, cushions, posters and ornaments, filling the base with up to 16 decorations inside.
If two players mix records, their secret bases copy over, as well as the bases of anyone else they've mixed records with. Entering one of these bases reveals a trainer who represents another player, including their trainer name, secret base decorations, and Pokémon team at the time of record mixing without its nicknames. Once the player has beaten the Elite Four this trainer can be challenged to a battle once a day.
A trainer can run in any secret base, but cannot ride a bike.
A boy in Mossdeep City reminds a player of where their own secret base is located.
It is possible to do a legitimate form of item duplication using secret bases. A player merely needs to have one of their Pokémon hold an item before exchanging records. When their secret base appears on the other player's game, their Pokémon will be holding that item and it can be stolen with Thief.
Generation IV
In Sinnoh, there is a vast underground system of tunnels where players can, among other things, build a secret base by drilling into the wall. Instead of mixing records and challenging an NPC with the other player's team, however, players can find each other in the underground itself and race to capture each other's flags, if they are near enough to each other for DS-to-DS wireless communication.
