Mt. Aleph (アルファ山 Alpha Mountain) is a dormant volcano in northwestern Angara. Within it is the sacred Sol Sanctum, ostensibly a shrine to the sun (Sol) that the inhabitants of nearby Vale revere, denying everyone passage but the holiest priests, without really being aware of its true nature. Hidden so deeply in the sanctum that failure to disarm the traps will cause disaster, waits the Elemental Star Chamber, the resting place of the four Elemental Stars necessary to fire the four Elemental Lighthouses, that, when lit, will restore the awesome force of Alchemy to the world. Mt. Aleph is the most important location in all of the Golden Sun series.
The exact origins of Mt. Aleph are unspecified. When Alchemy was sealed away from the abusive tendencies of mankind, the process involved putting out the elemental beacons alight atop each of the four Elemental Lighthouses situated throughout the world - these beacons are the purified essences of each of the four elements that comprise all matter. These essences were placed within the four jewels known as the Elemental Stars, and these in turn were hidden away within the deepest chamber in Mt. Aleph. The chambers within the mountain are designed to keep out those who are not Adepts, while allowing those who are to take the Stars and cast them into their respective Lighthouses, returning the power of Alchemy to the world.
A town that emerged at the foot of Mt. Aleph, Vale, developed a cultural reverence for the mountain because of the influence exuded by the Psynergy stones within it, which granted them varying degrees of Psynergy. Their legends believe that Mt. Aleph, whose name supposedly refers to "the origin of all things", is the greatest source of Alchemy, and that Alchemy is the foundation of Psynergy (whether this is fully accurate is unknown). The villagers believe that Vale stays pristine because of "Mt. Aleph's purity", and that if said purity is tainted, "terrible retribution" is risked.
 In Golden Sun and The Lost Age
During the game's prologue, a raiding party of Mars Adepts from the far north led by Saturos and Menardi come to Mt. Aleph and infiltrate it in an attempt to take the Elemental Stars. However, they fall victim to the trap when they attempt to gain access to the Elemental Star Chamber, and all of the Mars Adepts perish while Saturos and Menardi barely escape with their lives. The trap causes a magically-generated thunderstorm to surround Mt. Aleph and Vale, which additionally causes a massive boulder situated somewhere on the mountain, to start descending towards the town. Some of Vale's residents manage to delay its advance with their Psynergy before it inevitably crashes down upon the village, causing the supposed loss of four individuals.
Three years later, the elderly scholar Kraden takes a group of young Adepts with him - Isaac, Garet, and Jenna - into an unauthorized investigation of Mt. Aleph's sanctum, which he claims is an important venture for the sake of their education as Adepts. (In actuality, Kraden wants to discover the secrets of Alchemy because he was ordered to by his effective master, Babi, the lord of Tolbi.) Secretly following them into Mt. Aleph, however, are Saturos and Menardi, who have returned to attempt another raid of Sol Sanctum for its stars, this time bringing two companions in Felix and Alex. When Kraden's group manages to solve the riddles of the Sanctum and find the Elemental Star Chamber without triggering the trap from three years earlier, Saturos' group makes an entrance and forces them to retrieve the Elemental Stars for them. Saturos acquires the Stars of Mercury, Venus, and Jupiter.
However, when Isaac removes the fourth star, the Mars Star, from its pedestal, Mt. Aleph suddenly begins to erupt as a volcano, for the four stars had been set upon the pedestals in a way that their powers originally kept the flowing magma underneath subdued. Saturos' group is forced to leave the Mars Star behind with Isaac and Garet, while taking Jenna and Kraden with them as their captives; they know that if Isaac and Garet survive, they will attempt to pursue Saturos to retrieve Jenna, and will bring the Mars Star with them. After Saturos' group flees Mt. Aleph, Isaac and Garet come face-to-eye with the monolithic entity that has slumbered within Mt. Aleph until this moment, the Wise One. The Wise One then teleports Isaac and Garet to safety so they can return to Vale, and uses his near-godlike power to redirect the volcano's lava flow away from Vale and save it from destruction.
The eruption of Mt. Aleph immediately begins to have a profound effect on the entirety of the world of Weyard; the mass of Psynergy stones that long laid hidden within the mountain are spat out and showered all over the world in small chunks. The stones draw out the Psynergy abilities previously dormant in people throughout the world that come into direct contact with the falling stones, causing them to become new Adepts. Wild animals unable to mentally handle the influence the stones exude are driven feral, making the world as a whole a much more dangerous place. Most spectacularly, the plain at the eastern end of Angara, Mogall Prairie, very quickly transforms into a forest filled with hostile monsters. Despite all this, Vale itself is unaffected by the eruption, save for periodic tremors and a brief heating of the river that runs through town.
From here on out, the events of Golden Sun and The Lost Age take place. Isaac and Garet lead a party in pursuit of Saturos' group as per the command of the Wise One, whom they are ultimately able to slay them in combat, although failing to prevent the lighting of the Mercury and Venus Lighthouses. Felix continues Saturos' quest with the remnants of his group. Much later, Felix's group discovers proof of a shocking fact about Alchemy; for all its potential danger, its presence is the effective sustenance of the physical world of Weyard, and the world has been physically shrinking and breaking apart over the ages as a direct result of its absence. When Felix and Isaac's parties meet in a dramatic series of events upon Jupiter Lighthouse, whereupon it is activated with the Jupiter Star, Felix is able to explain to Isaac in full why breaking the seal on Alchemy is necessary to prevent the world from a gradual, yet premature end, and Isaac agrees to join forces with Felix on the rest of the latter's quest.
However, even as Isaac and Felix's combined party moves to light the final lighthouse, they are unaware that they have all been used: Alex, who had long since went off on his own after helping Saturos' party light the Lighthouses, had all along desired the full power of Alchemy for himself. Alex returns to Mt. Aleph and begins climbing up its exterior to its peak, knowing that standing there when the last lighthouse is lit will grant him the benefits of absorbing pure Alchemy made real, at the heart of its power. The Wise One, fully aware of this and everything else, confronts the Adepts at the aerie of Mars Lighthouse over their decision to break the seal on Alchemy and free a power that would inevitably lead to wars and strife, and notifies them of Alex's plot; soon, he puts them through a dramatic and harrowing test for the right to light Mars.
Ultimately, Mars Lighthouse is lit, and the automated process shared between the Lighthouses and Mt. Aleph to bring back Alchemy to the world begins in earnest. The four Elemental Beacons atop each lighthouse shoot out beams of energy in tandem, which gather into the airspace immediately above Mt. Aleph; the golden ray of energy that forms as a result of the four beams merging is what is referred to as the Golden Sun - pure Alchemy made real, at the heart of its power - and proceeds to bathe Mt. Aleph's peak, as though the physical mountain itself is infused with the power of Alchemy. At the end of the process, the mountain glows a radiant gold, and Alex stands upon the mountain's summit as the bright light engulfs his body. Ecstatic over the success of his grand plan, Alex immediately attempts to summon a thunderstorm upon Vale and the foothills of Mt. Aleph to flex his newfound power, but is baffled when nothing happens.
At this point, the Wise One appears before Alex, explaining that although he did indeed gain bolstered power and life, he did not gain the fullest extent of the power of Alchemy from the Golden Sun as Alex had intended; back when the Wise One met Isaac and Garet in the Elemental Star chamber when Mt. Aleph was erupting, the Wise One predicted Alex's plan and and set in motion a measure to counter it by imbuing the Mars Star with a particular property, which as a result caused the Mars-aligned portion of the Golden Sun to enter Isaac's body on Mars Lighthouse instead of Alex's on Mt. Aleph. Therefore, Alex is not all powerful as he would have been otherwise. Believing the Wise One is trying to anger him, Alex attacks him with the power he has, but the Wise One telekinetically knocks him back, leaving him subdued.
Mt. Aleph and the surrounding landscape begin to quake and change form, and the mountain itself starts sinking into the ground; since Alex is unable to move, the Wise One explains that he may not survive if he is "swallowed by the earth", but ends by noting that if Alex does survive, they might meet again in the future. Alex presumably meets his fate when the golden mountain half-submerges, leaving its top portion above ground as a permanent monument while everything in its vicinity is reduced to rocks and rubble. The town of Vale is destroyed, though its citizens are notified by the Wise One beforehand and are able to evacuate safely.
The new, half-submerged gold-colored state of Mt. Aleph has been a subject of much debate on community forums since the release of Golden Sun: The Lost Age. It may in fact be the Stone of Sages talked about in-game (since it is said in-game that it's the light of the Golden Sun that "gives shape" to the Stone of Sages), meaning it would be the massive vessel that "dominates everything" for being what brings the power of Alchemy back to the peoples of the world. Mind Read dialogue in post-game Prox, normally inaccessible without a hacking device, suggests that a future conflict may revolve around people vying for control of the Stone of Sages.
 In Dark Dawn
By the time Golden Sun: Dark Dawn begins, Mt. Aleph is an active volcano, and because of changes in the landscape caused by the Golden Sun event, it is now inaccessible by land. At some point, the villagers of Vale resettled at nearby communities, with Kalay being the largest known.
In the meantime, Isaac and Garet have been observing Mt. Aleph from their cabin located on the nearby Goma Plateau. They make plans to use the Soarwing, an invention of Ivan's that allows an Adept to fly using Psynergy, to one day travel to the volcano. However, their only Soarwing is broken by Garet's son, Tyrell, when the young Adept tries to use it without proper training. While most of the repairs are simple, they lack a crucial element, a Mountain Roc's feather. Thus, Isaac and Garet send Tyrell, along with Isaac's son, Matthew, and Ivan's daughter, Karis, on a quest to retrieve a feather so that the Soarwing can be repaired as soon as possible. It is suggested that the urgency of the situation is because Isaac wants to fly to Mt. Aleph as part of his investigation into the mysterious Psynergy Vortexes that have recently appeared.
 Name origin
The name Mt. Aleph is stated by the legends of the people of Vale to refer to the origin of all things. Among Mt. Aleph's allusions to real-world culture is that its name is based on the first letter of the Hebrew alphabet, א, which is called an Aleph. The Hebrew connection is likely intended to indicate a parallel that can be drawn between Mt. Aleph and the Biblical Mt. Sinai, where the Ten Commandments were handed down by God; in Golden Sun, for comparison, the god-like entity the Wise One gives his commandment to Isaac while they are still in Mt. Aleph, which would set Isaac off on his quest. In the Japanese version, however, the Mountain is called Alpha Mountain, referring to the first letter of the Greek Alphabet, which is more thematically consistent with the classical antiquity aesthetic referenced by the rest of the series' setting - a flat-earth world where all matter is made up of the Greek classical elements. The Biblical allusion therefore pertains only to the game's translation.
- After the initial dungeon is completed, Mt. Aleph cannot be returned to even with the use of a cheat device, and is used exclusively in cutscenes. In The Lost Age a well known trick can allow the player to walk "into" Mt. Aleph, but there is no special code for the area; it behaves in the same manner that any ordinary part of the world map would.