To unlock the third island, Tum Tum Island, get a "Rainbow" rank on every level on both islands. Tum Tum Island is between the two islands on your map. When you select this island, you will be prompted to enter a five character alphanumeric code. This code is not case-sensitive. The code you enter will generate a new map using all maps from both islands, but with new waves of creatures. Every code will generate a different map, allowing over 7 million possible combinations. Some codes for maps are as follows:

12345
BEEGE
BITCH (very difficult)
BRAIN
BUTTS (difficult Racer Gardens variation)
HAPPY
HORNY