The island across from the Hacienda is a little island near the Hacienda del Monte-Pelado, in Desert Island. Unnoticed by many of the islanders, the little ittland can be seen from a telescope in the roof of the Hacienda. In the island there is a big, flooded cave, which can be entered to access some kind of lost temple on the inside. The temple is protected by skeletons and traps, and holds the Protection Spell in the last room, guarded by a monster.
After seeing the island with the telescope, Twinsen can tell the Dino-Fly to take him there. Once there, he can enter the cave with the help of the Proto-Pack, and go through the temple, finally getting the Protection Spell.
The cave can also be reached from the shore just outside Hacienda using the Proto-Pack.