Shark Island is a location that appears in the Adventures of Sonic the Hedgehog television series. It is a small landmass on the Mobian Ocean on Planet Mobius. It first appeared in the episode "Black Bot the Pirate".
Robotnik, Scratch and Grounder traveled back in time 300 years to obtain the Chaos Emerald of Invisibility, which was buried on Shark Island. After landing on Blackbeard's ship and roboticizing the pirate, Robotnik took command of the vessel and set sail for the island. The island was covered with dense forest and laden with all manner of booby traps (even though the island appeared to be uninhabited and whoever set them up is not explained), though Scratch and Grounder only managed to set off one.
Robotnik lost the Emerald of Invisibility to Sonic, who threw it through the Time Warp.
Robotnik returned to Shark Island in the episode "Prehistoric Sonic" (presumably at a point earlier than his original visit) to re-acquire the Emerald of Invisibility and found it before Sonic could stop him. He then left Sonic and Tails trapped on the island by using the Chaos Emerald of Life to turn an empty treasure chest into a crab, which ate Sonic and locked him inside itself. Sonic and Tails escaped Shark Island after Tails freed Sonic from the crab-chest by asking a female crab to seduce the crab-chest.
After the defeat of the Supreme High Robotnik, the Chaos Emerald of Invisibility was returned to Shark Island by Sonic and Tails's multiple selves.