Tunnel Ruins

The Tunnel Ruins is the set of underground passages connecting between the the ancient ruins north of Lalivero (upon which the Babi Lighthouse is being constructed in Golden Sun) and the true entrance to Venus Lighthouse. In Golden Sun, it is the second of a three-dungeon "set" that is progressed through as one final dungeon at the end of the game, and is accessed beyond the large red door at the cavernous base of Babi Lighthouse.

What makes the dungeon unique is that it forks to become two pathways, and which one the player would be forced to travel down is based on which of two possible configurations of the glowing circles on the floor in front of the giant goddess statue in Venus Lighthouse's exit portion display. In other words, it requires at least two visits in order to get the armor artifacts available along each path, and to toggle the pathway, one would have to go all the way back south to the external "exit" portion of Venus Lighthouse and interact with the giant statue.

Other than it being an area where both Saturos' party of Adepts and captives and Isaac's pursuing party of adepts go through to get to Venus Lighthouse, the Tunnel Ruins themselves are not the site of any story developments. Their existence is unknown to the people of Lalivero above ground until after the first game's finale, though, because they were never able to get past the red door at the base of the ancient ruins north of Lalivero.

Walkthrough
When you enter Tunnel Ruins from the interior of Babi Lighthouse, you end up at a room with a giant goddess statue on the wall which has an identical display of circles on the floor as there was in earlier Venus Lighthouse, and if the one in Venus Lighthouse was interacted with so that the circles were left glowing, the same configuration of circles will be lit up in this room. There are two different configurations of these circles (each circle represents a room, with the circle right in front of the statue's lap representing the room just south of this one): One where the circles in the center and right are lit up and one where the circles at the left are lit up, and these are the two separate "dungeon paths" through the area that connect to the final room that leads to Venus Lighthouse (as represented by the small Venus Lighthouse model at the last circle). If you're collecting items, you would want to explore both paths at some point.

The first room you get into when you exit out the south end features two pillars which you are to move around so that you can hop to the ledge to the right and reach the room's exit to the south. This puzzle is easier than it appears; using the Move Psynergy on the right pillar to move it down one space and right two spaces is all you need to do to reach the next room. The next room below is the one that changes based on which dungeon path you've selected; there will be a series of hoppable platforms leading left if you've chosen the left path, or there will be a series of hoppable platforms leading down if you've chosen the center path. The two lead down to their own doors.

The left path
The first room of this left path has two yellow-tipped pillars to move around similar to what you just did previously. Go down the stairs and use Move so that they are both at the left end of the moveable area marked by dotted lones, with one pillar at the top left corner and the other at the bottom left corner. Use this new hoppable path to exit out south.

The second room has various white-tipped pillars, and is more time consuming. Push the left of the two white-tipped pillars three spaces left, and move the right pillar three spaces left so that you can hop to the bushes at the upper left. Remove these bushes with Whirlwind to expose a third such pillar, and now Move it one space left and push it down off the elevated portion of floor onto the ground below. Push this pillar along the long, winding way to where its path eventually ends next to the wall slide, then move the upper right pillar three spaces back right to construct a new hoppable path. Whirlwind the bush covering the door to the lower right to enter it. Follow the linear path to the next puzzle room.

At a room where there are three gray cubes that may be carried around with the Carry Psynergy, move the rightmost cube one space left, then use Carry to drop it one space south so that it lands on the white platform below; like a scale, it will cause the white platform to the left of that to go up. Do the same thing with another carry cube, dropping it onto the top of the other carry cube to further propel the left white platform up. Ignore the remaining third carry cube, as now you can hop and go out the room's south exit.

In the horizontal hallway-like room you end up in, hop left and go down into a room containing a statue. Go to where it is, and cast the Reveal Psynergy to see it replaced by a treasure chest, which contains the armor artifact Oracle's Robe - unless you somehow manage to get two Feathered Robes as random drops from Wild Gryphon enemies for both Ivan and Mia, this is ideal for Mia to wear. Go back up, hop right, and go down the other doorway leading down to get to the colored statue room right before Venus Lighthouse.

The center path
Immediately in front of you in the first room on this dungeon path is a small pillar containing water in it that can only be pushed right onto a floor tile with a water-drop symbol on it. When you do so, pillar will sink into the hole and the pillar in the abyss to the right will raise, so you can hop across it. Below that is the same type of pillar setup, but the pillar contains no water; this pillar will only sink into its water-marked tile if you cast the Douse Psynergy on it to fill it up with water. Once you've solved that, go out the door at the bottom left.

The next room has a series of hoppable platforms that branch down and right. Go right first, for it leads you to a room with a statue; cast Reveal around it to find a treasure chest containing the Asura's Armor, which should likely prove useful for either Isaac or Garet. Return to the hoppable platforms and go down, and exit into the next room.

This room that has an empty pillar that can be moved to the right in a "U"-shape might be tricky for some to figure out on their own, as there is nowhere else in the game that demonstrates that when the top part of an empty pillar is filled with water, that tile of water is then treated just like the many puddles throughout the game that turn into ice pillars when Frost is used on them. Move the empty pillar to the right enclosure, cast Douse to fill it up with water, then cast Frost on the water to create an ice pillar on top of the other pillar to essentially create a pillar that is much higher. This new taller pillar can be used to hop onto the ledge to the right, and now the lower exit is accessible. In the subsequent room, hop left and go down the door below to get to the colored statue room right before Venus Lighthouse.

To Venus Lighthouse
The room at the bottom end of Tunnel Ruins features five statues of separate colors and five tiles on the ground; you're supposed to move the statues onto the tiles in the right way, and the clues as to where the statues go in relation to each other are provided by using Mind Read on each of the statues (e.g. "Red shines northwest of me."). The correct configuration is for Red to be on the upper left tile, Blue on the upper right, Yellow on the center tile, Green on the lower left tile, and Violet on the lower right tile. The door to the right of the room opens when all five statues are at their correct tiles, and when you go up the subsequent room's staircase, you will end up at the beginning of the true dungeon portion of Venus Lighthouse.

Collectibles
Asura's Armor - In a hidden chest in a room accessible from the center path.

- Oracle's Robe - In a hidden chest in a room accessible from the west path.