Please, kindly note, that there're different islands in the game. Some of them become available with your progress in the story and some of them are open only for a short period of time. Here you can find more information about different islands in the game.

Sometimes you may notice that an island that you were exploring just yesterday suddenly dissappeared. There is a reason for this. Treasure islands and all the event islands are only available for a certain period of time. You can always see the remaining time on the icon with the islands tasks on the right side of the screen.

Once the time is up, the island dissappears. And what is very important, all the resources that you left on the island will dissappear with it. So, please, make sure you don't leave anything valuable there!