Its existence was revealed in a conversation between Tommy Vercetti and Avery Carrington, which was removed from the final release of the game. Following is the conversation between Tommy and Avery:
Avery: Hey Tommy, it's Avery! Now listen, I got me all tied up at the moment and I have a representative of mine needs chaperoning out to the Gator Keys. I'm after some land out that way, so I'm sending someone out to sweeten the deal. Could you do me a favor and make sure he gets there ok?
Tommy: Yeah, sure thing Avery. Where'd you want me to pick him up?
It is plausible that the Gator Keys featured Crocodiles and swamps in the game.
These keys are considered to be owned by Avery Carrington, a big time real estate mogul and conspirator, and it is assumed that Panlantic may be involved in this project. Therefore, Tunnels are assumed to be one of the paths, leading to the keys.