Bilibin Cave

Bilibin Cave is a cave trail in northern Angara, allowing passage between the heartland of Angara and the continent's snowy north region. It is so named because it is northwest of the town Bilibin, but many mistakenly call it "Imil Cave" because during the main journey in Golden Sun, the party travels through here to get to the town of Imil and the Mercury Lighthouse itself.

It is not a site where much story takes place in the series. Any travel between Bilibin and Imil would take place through here. The group led by Saturos and Menardi come through here on their way to Imil, to activate the Mercury Lighthouse's beacon, and the group consisting of Isaac, Garet, and Ivan travel through the cave in pursuit of them shortly afterward. Mercury Lighthouse is eventually activated. Shortly afterward, Saturos' group travels back through the cave to return to the rest of Angara and continue towards their next destination, Venus Lighthouse. Isaac's group does the same, with one more in their group: Mia, from Imil.

Description
Bilibin Cave is the dungeon-style cave path that allows passage from the main majority of Angara's landmass to the snowy region of Angara up north. In Golden Sun, the player's party can through it after crossing the similar Goma Cave and stopping by the town of Bilibin to stock up on better equipment. It is recommended that players complete the Kolima segment of the game east of Bilibin first, as the Imil/Mercury Lighthouse segment that lies beyond this cave is much harder and is generally meant to be played after Kolima, but you can intentionally go this way first for a specific challenge.

Walkthrough
When you first enter the location on the overworld, you will arrive at Bilibin Cave Entrance. Head up to see that left of a river you can hop across is a sprout on the ground. While the other side of the river features the "standard" entrance into the cave itself, the sprout can lead into a superior "alternative" entrance into the cave. It can become a climbable rope of ivy if you cast the Growth Psynergy on it; to access the Psynergy, set a red Mars Djinni on Isaac and/or a brown Venus Djinni on either Garet or Ivan. Climb it to reach a dead-end to the upper left which can be hopped to another cliff path to the right, and slide down to a cave entrance to the upper right of the standard entrance. When you enter through the alternative entrance, you will get to a chest containing the Elven Rapier, a weapon that should be superior to anything else you hold at that point. Note the four rocks surrounding the nearby wall; there is a hidden doorway here that can only be entered much later in the game, once you have the Reveal Psynergy. Slide down to the ground below and you will be at the cave trail one traverses normally.

Inside the cave, at the trail you would end up in if you take the normal cave entrance, follow the path left and up, and at the upcoming fork take the right path instead of the upper one. You will enter another cavernous area with ice-like objects scattered about the ground. There is nothing you can do about these objects, so just follow the path until you pass between a pair of torches lined up horizontally. Push the right torch one space down and two spaces right to melt an ice pillar into a puddle, which you can hop over. Follow the subsequent path around to a chest containing a Vial. Retrace your steps, hop back over the puddle, and head downward to exit into the northern area of the overworld, where the eventful Imil and Mercury Lighthouse segment of the game awaits. After completing all that, you will pass back through this cave with Mia in your party to return to the Angaran mainland, and finding your way back down through the cave shouldn't be hard.

It should be noted that the monsters in this cave are a good deal stronger and more rewarding than any of the monsters east of Angara in the Kolima region, but the monsters should still be quite easy to deal with even for parties that have just crossed Goma Cave. This makes Bilibin Cave an excellent place to level up early in the game, and all the coins accumulated through battle can be used to stock up on more of the stronger and costlier equipment sold at Bilibin. If you decide to play through the Imil segment before the much easier Kolima segment, this development will be extremely beneficial, but even if you are playing the Kolima segment first, this development will make that event progress much easier. Whichever gameplay path you choose, you should come here first to at the least grab powerful Elven Rapier before setting off towards your intended destination.

Much later in the game, after crossing Lamakan Desert to get to the southwestern Kalay region, you have the opportunity to revisit the towns and areas you previously explored and undertake various sidequests. You can return to Bilibin Cave with the Reveal Psynergy in Ivan's possession to explore the rest of the cave you couldn't before. First, enter the cave through the alternative exit reachable with Growth, and go to the wall behind the chest that contained the Elven Rapier. Cast Reveal and you can enter the door into a hidden cave path. Two torches, one with its flame on the ground in front of the next doorway, block the path. Cast the Douse Psynergy on the ground flame, and enter into the icy cave area in a higher level than usual. Walk to the left and hop to the left, and walk to the upper right until a small flame on the lower ground left of a puddle is below your position to the right. Cast Douse here to put out the flame, and the puddle will automatically freeze into a pillar which you can hop across. Enter the cave up at the end to get to two chests containing a Psy Crystal and a Water of Life.

Bestiary
(Note that the Troll, Rat Soldier, and Drone Bee appear only in the first area inside the cave itself, in addition to the Ghoul, Gnome, and Ooze; otherwise, the latter three appear throughout the entire cave.)

Collectibles
Elven Rapier: Found on a chest in an elevated portion of cave accessible only by entering the cave through the second "secret" entrance. Vial: Found in a chest near the end of the cave trail, push a torch next to an ice pillar to melt it and access the chest. Psy Crystal: Found in a chest in a hidden cave accessible only with the Reveal Psynergy. Water of Life: Found in a chest in a hidden cave accessible only with the Reveal Psynergy.