The translation key from Professor Hammerhead's journal provides a way to "OPEN" the undergound door in the ruins.
By pressing the symbols that correspond to O-P-E-N on the face (and then the nose), you will open the door. (These would be the 3rd, 4th, 6th, and 7th teeth starting from the left side.)
you get a paper
go up the rope in to the hot air balloon
on top of the building
On top of the cave
on the roof of the ancient ruins
When you jump on the vines at Shark Tooth Island, go on a vine very high but dont run into the coconuts!Then jump on the cave to the ruins. At the top, you will find the paper.
on top of the bulding where the cocanuts fall
When you jump on the vines at Shark Tooth Island, go ona vine very high but dont run into the coconuts!Then jump on the cave to the ruins. At the top, you will find the paper.
When you come back from the medicine man on the deck of the tree jump really high so you can reach the roof of the underground thing. Then you go underground on the machine you translate OPEN in the translation on the paper.
You keep it to translate the symbols for the underground door. (see related question)
just go to poptropicasecrets.com then click walkthroughs, then find sharktooth island by scrolling down, and it will tell which buttons to push.
You get the translation paper and go to the cave its on the top left of the cave.