The island used to be accessible through a dead tree in the northeast most corner of the rotworm wing of the Fibula Dungeon. What you would find on the island is a Giant Spider and a house. At that time there was only one island where there's two these days.
Nowadays the building and the Giant Spider are removed from the island and it's inaccessible by normal players (you can only teleport there)
From what i read on forums people are still looking for the way to get there, because they think its a new "Tibia secret". But offical this island is closed for players.
Calcanea with house:
On youtube you can find few videos about the island - it shows how you could get there before - example
Forum (from 2008 in pl) - Tibia.pl
TibiaWikiBR - here : "Contudo, especula-se a possibilidade de algum outro acesso, ainda não descoberto." - "However, it is speculated that some other undiscovered access is still available."