The Earth is the starter location for all sharks. It can be found when you press "Play" on the Menu or go to the portal in the Alien planet.
The location that Hungry Shark Evolution takes place is called Skull Island, as named in articles shared on Hungry Sharks community pages. The Mayor is called Mayor Bobby, who assures the humans there that it is safe to swim. There are regularly strange sightings and discoveries by the people there that often lead to the introduction of a new shark in the game. Due to lack of homes and food (besides Artic Land) it's unknown where and how they live.