Idejima

Idejima is a peninsula initially located at the eastern region of the continent of Gondowan, specifically an area only accessible by passing through a cave connected to the Gate in the Suhalla Range.

Role in the story
Some time after their initial, unsuccessful, raid on Sol Sanctum in Golden Sun's prologue, Saturos and Menardi, along with their new party members, Felix and Alex, return to Gondowan, anchoring their Lemurian Ship at Idejima before they undertake their long journey north to Angara (presumably because no suitable ports could be found closer to Vale). This boat remains here for a long time and eventually, Isaac's party's travels across Weyard brings them through the Suhalla Gate. If the player goes through the cave onto Idejima, they will trigger a cutscene where Iodem mistakes this boat for Babi's one, and attempt to use the Black Orb on it, to no avail (later it is revealed that Babi's ship is in fact moored within Lalivero).

As Isaac and crew climb to the Aerie of Venus Lighthouse, Alex, Jenna, and Kraden escape from the bottom and make their way to Idejima, where they witness the reactivation of the beacon by Saturos and Menardi. However, the earthquake generated by the release of Venus Psynergy breaks Idejima off from the mainland to form its own island that actually starts to float eastward, into the Great Eastern Sea. Shortly afterwards, Felix and Sheba, having jumped and fallen (respectively) off the top of the Lighthouse, wash up on Idejima's shore.

Some time later, as Idejima floats along the ocean, it passes close by the continent of Indra, but if left to drift on its own, would have missed landfall, much to Jenna and company's dismay. Fortunately for them though, is then that a great a tidal wave is generated from the Sea of Time in the center of the Great Eastern Sea, which crashes into Idejima, driving it into Indra's north end and subsequently moving the entire continent, wedging it between Gondowan and Osenia. Gradually, Felix, Jenna, Sheba and Kraden wake up and form their own travelling party, wherein they begin their journey as the main characters of Golden Sun: The Lost Age.

After the prologue of The Lost Age, Idejima ceases to be its own unique area and simply becomes part of the overworld, furthermore, Menardi's ship is not seen again, presumably being destroyed in the collision with Indra (even if it weren't, it is inoperable anyway, as the Black Orb needed to operate it was destroyed atop the Aerie).