Gondowan Cave

Gondowan Cave is an underground cave trail that connects the southwestern portion of Angara with the northeastern portion of Gondowan. From a gameplay perspective this is the path that allows players of Golden Sun to travel in between the two continents after having sailed across the Karagol Sea, making it very important to go through for the sake of completing side-quests in Angara at a later point. Gondowan Cave can be passed through and fully explored as soon as you arrive at Gondowan by the ship, since it only requires the Lift and Move Psynergies.

Walkthrough
Head East from Tolbi and the Passage to reach Gondowan Cave. This is an alternative route to return to Angara, since you can't take a boat back across the Karagol.

Upon entering, head up the left path and through a door. Use Lift on the boulder that is in the way, then continue down the path, entering a door facing north. Keep walking north on the path until you see several platforms in the water. Jump right to reach a treasure chest containing a Lucky Medal, then keep jumping right and finally north. Be sure to go back and get the chest that was on the northern path, because it contains an Apple. Go through the door on the right heading south, then climb up the stairs. On this upper level, use Move to push a stump out of your way, into the water. This will simplify any extra voyages through the caves you may make. Continue heading right until seeing a large stump, which needs to be Moved twice. Now, just walk south and out the door to get back to Angara.

Collectibles
Apple: On the upper half of a room, with a chest containing a Lucky Medal on the lower half of it. Lucky Medal: On the lower half of a room, with a chest containing an Apple on the upper half of it.