Secret Bases are a customizable space that builds on the bedroom concept from Gold and Silver. Using the move Secret Power (TM43), you can turn special areas on the map into little forts where you can place items such a Pokedolls, carpets, etc. You can get this TM for the first time by talking to a boy at the topmost point of Route 111, where he demonstrates on a Secret Base tree. By mixing records with your friends you can visit them in their Secret Bases on the map and even battle them once per day.

There are three types of Secret Bases:
Trees: This is the type that initially demoed to you on Route 111. They are trees that look a bit different than the rest on the map. These trees can hold two secret bases, one on either side, even though you can only have one at a time.
Caves: Small square indents on the side of rock walls can be converted into Secret Bases too. Note that these are never within caves proper, and only appear on the outside map. Many of them require boards to patch up holes in the floor.
Bushes: Big green clumps of grass. Some have room for two bases, and many require the Acro Bike to access. These are the rarest kind of base, with just a handful existing in the routes around Fortree City.

You can put up to 16 decorations in your base at any time, and these vary greatly, from posters to dolls to even tents. There is a boy in Mossdeep who can help you recall where your Base is should you forget. For a list of decorations, click here.