Mercury Lighthouse

Mercury Lighthouse is the Elemental Lighthouse of Mercury (Water), located in the wintry northern region of the continent of Angara. It is the first Lighthouse to be visited and explored in Golden Sun, and therefore the series as a whole, and is explored quite early on. It is the first highly challenging and involved dungeon the player contends with in relative terms, containing a highly difficult relatively early-game boss battle; while technically it can be explored and played through before playing through the easier Kolima Forest/Tret Tree dungeon set, this is not recommended for players wanting fair challenge. Mia joins here as a party member, and after the storyline events at the top of the lighthouse, you may collect the important Hermes' Water item, which is needed on a return visit to Kolima Forest.

While in the Lighthouse, Mia receives 4 PP after every round in battle.

Walkthrough
If before your encounter with Mia in Imil you come here, you'll find that the only thing that can be inspected is a glowing blue wall blocking the main entrance into the main structure, so return here only after Mia decides to go off to Mercury Lighthouse herself. Once she is here, after talking with her, stand to the right of the statue now blocking the door and use the Move Psynergy to move it one space left, and a cutscene will transpire where Mia will talk about the Ply Psynergy, which is the main Psynergy you would use to solve puzzles in the Lighthouse. Then follow Mia inside, where you find her being blocked off by a Lizard Man monster. Simply walk up to it and engage it and defeat in battle, and Mia goes forward again. The next series of rooms only lead you north and up a stairwell, and eventually you get to another room where Mia is impeded by an obstacle - that of a blue statue on the opposite end of a gap. Use Move to move the statue one space left and make it fall into the gap, and after another cutscene, Mia joins as a permanent party member (all throughout Mercury Lighthouse, you should notice that Mia regenerates 4PP at the end of each turn of battle). Exit through the upper right door.

Pass by the hallway with three waterfall walls and go down the lower right exit into a mazelike room with rollable sections of pipe (mercifully free of random monster encounters). Roll the disconnected portion of pipe right to cause water to flow, and go down and touch the stream of water shooting right to be propelled right. Go down the stair path leading under the pipe to the right, go down and let the stream of water shooting down propel you down, and go left and up and under the pipes via the stair path leading left to right. Go all the way up to the top right corner of the room, go under the mass of pipes through the stair path left, and push the pipe section you rolled earlier back left to turn the shooting streams off. Now go down, and here is a branching fork to consider: go right and back through the stair path to the right to get to the lone chest in the room, which contains a Psy Crystal. Return to the fork and go down, left, up, and make a U-turn to go through the stairway path at the bottom, and exit the room out of its bottom end.

In the next room, hug the right end of the wall as you hop across the immediately visible crumbled area of floor, and follow the elevated path along the bottom length of the room until you get to an area with a pushable statue. As should be obvious, you're supposed to push the blue statue to the right over onto the blue floor switch to the left along the path designated by the dots on the ground, and that will keep the door above open. Slide down the wall slide to the immediate lower right and go up to enter the now-open door, and slide down the slide into the hole to fall into the lower floor onto a tile at the center of a body of water.

After a spiritual message intones He who honors the heart of the goddess shall stand at the center of all, like a swan in the center of a rippling pond, hop up onto the lap of the large goddess statue and select the Ply Psynergy; rather than the selection for which character you want to use it on in any other circumstance, Ply will instantly be used as a Utility Psynergy effect, and the result is that all tiles in Mercury Lighthouse like the one you landed on will be "turned on" with a new magic effect: It gives you the ability to hop across the surface of the water up to four times in a row. If after the fourth hop you are not back on solid land, you will fall into the water and reappear at the last point of solid land you were standing on. Test this trick out by hopping to the right side of the room and going through the upper right door.

At the right end of the area of the narrow room you enter into is another one of the glowing water-skipping tiles. Hop onto it, then hop right three times and hop down once to access a new room off to the lower right - you will end up walking out of a waterfall wall, which should instantly clue you that throughout the game, falls of water may conceal hidden entrances. When you open the chest to the right, you will find it is actually a monster called a Mimic, which rewards you with the item it contains after defeating it in battle, in this case a valuable Water of Life. After the mimic battle, walk up against the waterfall wall to the right of the room to enter a hidden room containing a chest that can easily be hopped to and from with the water-skipping tiles; it contains the Psynergy Armor, which features a very useful increase to the equipped Adept's Psynergy Point meter. Return to the waterfall room, return to the previous room by walking through the waterfall wall to the left, and hop back to the central part of the room with the water skipping tile. Enter the door to the north to reach the largest room of Mercury Lighthouse.

In the large room where you originally went up the stairs in the center, hop onto the water-skipping tile below that, hop right three times and hop down to another tile. Your "hopping path" "branches" from here; to get a treasure, hop up three times and right once, hop left once and up three times, and hop left to get to solid land where a chest contains a Sleep Bomb. Hop back down to the land around the central stairway with the help of the water-skipping tile to the right of the chest. Water-skip your way back to the tile that has "branching hop paths", and this time hop right three times, hop up four times, hop up three times and left once, and hop right once and up three times to reach the solid land at the upper right corner of the room. Use the water-skipping tile here and hop left three times and down once to reach solid land, and follow it left and down to another water-skipping tile, which you are to hop south from this time. Exit the door at the bottom left corner of the room, and in the room below, hop straight down across the water-skipping tile and enter the room below that.

Now in another room with waterfall walls, press up against the center of the three falls to enter the room behind it, and go up into the upper right door to enter another room that has a rolling pipe puzzle. There are three disjointed sections of pipe here; go up and push the upper horizontal pipe up, push the lower horizontal pipe up, and push the vertical pipe left to complete the water circuit; as the game goes out of its way to show you, the previous room is filled up with water. Return to the previous room and use the water-skipping tiles to reach and enter the stairwell to the upper left of the room, go through the subsequent blank room up another flight of stairs, and when you get to a room with a door and a floor switch, ignore these as you go up yet another flight of stairs.

You are now in another random-battle-free room with a pipe-circuit puzzle, and the aim this time is to cause the different water spouts pointed at the statue to project the statue into the square hole in the ground. Go left along the left edge of the room and take the long stair path under the three pipes to get to the upper right side, and standing right around the squarish tile object to the right, press left to roll a vertical section of pipe left - the blue statue will be shifted left. Go through the small stair path and push the pipe right again - the blue statie will be shifted down. Go all the way around through the long stair path and push the pipe left again, and this time, go up and roll the horizontal pipe section down. Push the vertical pipe section right once more and it will collide with the horizontal one, and the resulting circuit will shift the blue statue right into the hole. Go back downstairs to where the statue now is, and push it onto the floor switch so that the door above will remain open for you to go through.

In the next room is a series of statues lining the wall similar to what was seen in Sol Sanctum, and push the third statue from the left out of the way to enter a room where there is a chest containing a Nut. Go back down and hop and climb your way over to the lone statue to the upper right of the room, and use the Move Psynergy to remotely shift it out if the way to go into the next room. Here, simply push the horizontal pipe up to force a portion of wall left so that you can enter the newly revealed door. In the subsequent hallway lined with waterfall walls, enter the fourth wall from the right to find a hidden room with a Mercury Djinni, which you must battle to earn. Once you defeat it and earn it - the Mercury Djinni Sleet - set it onto whoever already has the Fizz Mercury Djinni set on them to upgrade their class.

Exit the waterfall-lined hall out of the lower left, and the next few rooms lead you on a linear path. When you get to another hall lined with waterfall walls, enter the rightmost waterfall to get to a small room with a lone statue. Hop to the upper right and use Move to move it one space left, and open a stairwell. From there, follow another linear path to a room with a waterfall wall and a rainbow, and a spiritual message intones, He who honors the goddess of rainbows shall be guided to the heavens upon wings of fluid grace. Save here, then cast Ply on the blue statue to trigger a cutscene where you are transported to Mercury Lighthouse Aerie. Use your PP to heal everyone, then use the Psynergy Stone to replenish all PP, then hop left and make your way to the glowing sphere; when you do so, a substantial cutscene triggers, which eventually leads to a boss battle that is far more difficult and lengthy than any battle before in Golden Sun; to put its difficulty in perspective, the next boss battle in the game way over in Mogall Forest has less HP.

When you manage to defeat the boss, another lengthy cutscene ensues, at the eventual end of which you regain control and are directed to step on the lighthouse "elevator" to the right side of the screen; hopping onto that will transport you back down to the base of Mercury Lighthouse. Several NPCs are here around what is now suddenly a flowing fountain filled with healing water. If you did not collect the Empty Bottle item from Imil previously, return to Imil (which, incidentally, now has its vendors fully open) and collect the bottle from the chest in the first house you see, because you are supposed to stand in front of the point where the water from the fountain is flowing out, interact with it, and use the Empty Bottle to replace it with the Hermes' Water. While Hermes' Water can be used for yourself as a healing item equivalent to a Potion, and will replace itself back with the Empty Bottle and can be refilled, it is not meant for yourself; you're supposed to use the water in Kolima Forest to progress through the game. At any rate, with the Hermes' Water in tow, Mercury Lighthouse is complete.

Collectibles
Sleet: Behind one of the waterfalls among the wall of waterfalls in one hallway later on in the dungeon, right after the room with the chest with the nut. Run against each fall by pressing B.

Nut: In a chest in a room behind a statue that can be moved aside. Psy Crystal: In a chest in a room with a water-and-pipe puzzle, on the right side of the area. Psynergy Armor: In a chest in a room hidden behind a waterfall in the room where you fight the mimic. Sleep Bomb: In a chest in the large watery room with many magic tiles that allow you to hop across the water.

Background and story
The Mercury Lighthouse existed in Weyard's Ancient Past much like the other three Elemental Lighthouses as a source of its respective element in Psynergy and Alchemy. The structure is designed to work with the Elemental Star jewel of the Mercury element, the Mercury Star; if the Mercury Star, the container of the purified essence of the Mercury element, is cast down the well at the top of the Mercury Lighthouse, its essence would be channeled by the Lighthouse's internal mechanics and be placed atop the aerie as a spherical beacon emitting Mercury-based energy.

The location of the town of Imil was once the home of the Mercury Clan, who were the guardians of Mercury Lighthouse, and the modern-day residents have had legends passed down that the Lighthouse is to remain dormant. They also have the legend, however, that if the Lighthouse is lit once again, the empty fountain at its base will be perpetually filled with a mystical healing water referred to as the Water of Hermes, which has the power to stave off all illness and ensure physical health. As an Elemental Lighthouse, it may not be entered without the presence of a Mercury Adept with access to the healing Psynergy power of Ply, such as Adepts descended from the Mercury Clan bloodline.

In Golden Sun, sometime after the eruption of Mt. Aleph, a group of Adepts trying to light all the Lighthouses in the world with the Elemental Stars, led by the Mars Adept Saturos, arrive at Mercury Lighthouse, and the Mercury Adept Alex, one of two known remaining living descendants of the Mercury Clan, grants them entry inside. Alex has since decided to betray his bloodline oath of ensuring the Mercury Lighthouse remain unlit, seeking instead to have the power of Alchemy returned to the world because he considers it magnificent. Alex and Saturos set up obstacles behind them to block off entry, though, because Alex knows that the other known descendant of the Mercury Clan, Mia, also has the ability to enter Mercury Lighthouse and therefore potentially oppose them - and since the Lighthouse briefly glowed when Alex used his Mercury power to grant Saturos admittance, he knows she is sure to investigate.

Mia is unable to get past the statue that was set up by Alex to block the main entrance. Shortly afterward, however, the small group of young Adepts pursuing Saturos to prevent him from lighting the Elemental Lighthouses, led by Isaac, arrive and use their Psynergy to move the statue aside. Mia is tremendously taken aback to find that monsters from the outside world have since swarmed and take up residence, but when Isaac's group fights off the ravenous Lizard Man blocking her path, and later clears away more obstacles set up by Saturos, Mia and Isaac agree to join forces and climb through Mercury Lighthouse together.

Saturos and Alex's group reaches the Aerie of Mercury Lighthouse and cast the Mercury Star down the well of the Mercury Lighthouse, and soon, the swirling Mercury Beacon is alight. As they begin to leave, Isaac and Mia's group is suddenly transported up to the Aerie, and Saturos and Alex immediately hide behind the Mercury Beacon as Mia is devastated to find that she had failed the one duty placed upon her as an Adept. All of a sudden, they see the captives Jenna and Kraden being taken away from Mercury Lighthouse by Menardi and Felix, while Saturos makes his presence known and commends Isaac for having survived the eruption of Mt. Aleph. Saturos announces his intention to slay them all in battle and relieve the Mars Star in Isaac's possession by force, but suddenly realizes that due to his proximity to the Mercury-based influence of the Mercury Beacon, his Mars-based powers are inhibited. Nonetheless, he remains confident enough to engage the Adepts in a four-on-one battle.

Though Saturos proves to be a fierce and powerful warrior Adept even by himself with a cap on his power, the four Adepts eventually manage to best him in battle and render him barely able to stand. It is then that Alex, who was watching the battle from behind the beacon, makes his presence known, and Mia is thunderstruck. In response to his betrayal, Mia calls him mad, but Alex, stalling for time while Saturos gets his second wind, subtly attempts to convince Mia that a power such as Alchemy, reflected in how the Mercury Beacon allowed her to use Mercury Psynergy without depleting her reserves during the battle, is not something to just keep sealed away. Alex also explains in detail how and why the Mercury Beacon prevented Saturos from being able to use his full Mars-aligned powers. When Saturos is finally up, Alex uses his exclusive Warping Psynergy - a very advanced maneuver not even Mia knew Alex could use or can use herself - to bring himself and Saturos away from Mercury Lighthouse to rejoin Menardi and the others, but not before confirming Isaac still has the Mars Star with him.

A dispirited Mia resolves to prevent Alex and Saturos from lighting the rest of the Elemental Lighthouse Beacons and officially joins Isaac's traveling party of Adepts. When they ask her whether it is okay for her, as the town healer, to leave Imil, she assures them that it is all right now; she knows from the legends passed down that now the Water of Hermes is available through the newly flowing fountain at the base of the Lighthouse, and that the town will no longer fear epidemics and ill health from here on out. Indeed, people from Imil have already come to drink it, and all of the cold and flu epidemics that wracked the town before Mercury Lighthouse's activation are now gone. Mia and Isaac's group take the elevator down to the base and leave Mercury Lighthouse, resuming their quest in pursuit of Saturos.

At the end of Golden Sun: The Lost Age, all four Elemental Lighthouse Beacons are lit, which automatically triggers the process shared between Mercury Lighthouse and Mt. Aleph that brings the force of Alchemy back to the world. As though the Lighthouses are linked, they shoot beams of purified energy from their respective beacons and merge them in the airspace above Mt. Aleph, forming the Golden Sun. Prior to this, however, Mia's two young apprentices in Imil, Justin and Megan, are instructed by the Wise One to come to Mercury Lighthouse so that they may telepathically speak with Mia and her companions at the top of Mars Lighthouse long-distance. They tell her that the Wise One told them to tell the Adepts that when the Golden Sun forms, the energy that would be released by all the lighthouses simultaneously would make them quite dangerous, so Mia's group is to evacuate Mars Lighthouse posthaste.

Trivia

 * After acquiring the boat in Golden Sun: Dark Dawn, a lit Mercury Lighthouse is visible on the northernmost part of Angara. This is the only Elemental Lighthouse seen on-screen by the player's party in the game, which takes place thirty years later. One cannot visit it, though, as the landmass it is on is inaccessible. As pictured, it is within the field of darkness of the Grave Eclipse.