One day in summer, you land at C-Island where Dr. Jones has his laboratory...

Go to Coralcola

Go west and go through the path in the palm trees to reach Coralcola.

Talk to Everyone in Town

You won't be able to progress until you talk to everyone in the village. (You don't have to talk to the pig, though.)

Talk to the Chief

If you haven't already, go into the big hut in the northwest part of Coralcola and talk to the chief. Say Yes when he asks if you will help. He will give you a star (known in older versions as a yoyo).

Talk to the Tunnel Guard

Talk to the guard in the northeast part of the village. If you talked to everyone in town, the guard will recognize you and step aside. Go into the hut behind the guard and go down the stairs. There will be a cutscene, then the game will save. Then you will proceed to the cave under Coralcola.