Hunting for Triforce Chart 2, we encounter a need to gain entry to the cabana on Private Oasis. Thankfully, we quickly resolve said obstacle by simply becoming the new owners of the cabana and renaming the island SilentS's Oasis. Then, as we enter this new bachelor pad, we discover an underground catacomb area, filled with rats, sewage water, and...a couple of redeads. Lovely.
The rough, crudely drawn map I gave doesn't account for every twist and turn of the underground maze, but where there are forks in the pathways, that's mainly the use it has.