The Unknown Island is an uninhabited location that Bart and Lisa, along with their friends, were stranded on for an unspecified amount of days.
It first appeared in Das Bus when the kids had to swim towards the island after their bus went off the bridge and into the ocean. The island has large rocks as well as boulders that are clearly visible.
There are also mountains that are visible and even a cave in the mountains. These mountains are surrounded by trees and bushes. There is even a river that runs from the top of the mountains right down into the ocean.
A wild boar lived on the island until it was subsequently killed, cooked, and eaten by all the kids (sans Lisa who licked slime off of the rocks instead much to her dismay since she was the only one to do so). According to the narrator the kids were eventually rescued by Moe. It is unknown where the island is actually located at.