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