Taken from the wiki.
The Slime is the fifth hostile mob, and was added in Alpha. They appear as green semi-transparent cubes. Slimes come in four official sizes: huge, big, normal, and small. They are the first mob that can spawn in both light and dark areas. These creatures are only naturally found deep underground in the bottom 16 layers of the map. They usually spawn in large, cavernous spaces or in open mines. This is partially caused by the fact that a large space is needed for them to spawn. Small slimes apparently spawn in the peaceful difficulty.
Bold may be why they are so rare(?).
