There is an adventure linked to that tomb. (EDIT: Not an adventure, but a game play way of getting there without teleporting across or using a boat to get to that island.)
There is a place where I often send my sims to catch butterflies and fish at the little fishing hole. It's out in the world. Your sim needs Pangu's Axe which they find in China. If they have the Axe the sim can smash the boulders near that spot with the butterflies. There is a set of steps revealed under those boulders. The tomb is big and complicated but you will eventually come across a dive well that takes your sim to that tomb under that island.
I have a series of videos that do walkthroughs of the World Adventures Tombs.
In The Isla Paradiso Bunch Episode 35 you can see the boulders they need to smash to get into the tomb that leads to the tomb under that island. Later episodes of that video series will show how to get through that tomb and travel to the tomb you found under the island.
You can see the exact spot and the boulders that the sim needs to smash with the axe in this video at about the 3:08 position.
Location of boulders a sim needs to smash to find a large tomb in Champs Les Sims.
