Background and Prelude
Long before the event actually occurred, there was a Matoran legend, known as the Legend of Eternal Shadow, when the light of the Great Spirit will be lost forever. The sign of the legend coming true would be the twin suns of Metru Nui overlapping and darkening in a double eclipse.
Teridax, the leader of the Brotherhood of Makuta, an organization dedicated to serving Mata Nui and protecting the Matoran, had begun making plans to take control over the Matoran Universe, inspired by the League of Six Kingdoms. To do this, he planned to infect Mata Nui with a virus, causing him to fall asleep.
The Great Cataclysm
Mata Nui was making his way back to Bara Magna to fulfill his destiny and reform Spherus Magna. The virus then activated, and he went into a coma and fell towards the nearest planet, Aqua Magna. The twin suns of Metru Nui, Mata Nui's eyes, closed and darkened, forming a double eclipse. When Mata Nui hit the ground, the whole Matoran Universe suffered from an earthquake.
The earthquake of the Great Cataclysm caused some land to break away from the Southern Continent, and to blast onto the surface of Aqua Magna. How the piece of land managed to launch up onto Aqua Magna was unknown. This land would later be called Voya Nui, which would become a broken, stormy land only notable because of the Mask of Life, Ignika, in its large volcano.
The impact of Mata Nui's fall damaged his camouflage system, although it still functioned at the time, and caused the creation of the island of Mata Nui. Energized Protodermis also leaked when the camouflage system damaged, resulting in 6 regions to be created on the island, each with a different climate.
The earthquake also damaged The Pit, causing in the release of the prisoners, and also creating a hole in Karda Nui's ceiling that would allow Pit Mutagen to leak on the floor of the Universe Core, resulting in the creation of the Swamp of Secrets and the Stalactite Villages, resulting in the Av-Matoran returning to the Universe Core, and damaged other areas in the Matoran Universe.
While Mata Nui could not or would not prevent the devastating attack, he made sure that heroes would come to restore the worst of what was lost.
The most immediate matter was that the Matoran in Metru Nui had to be evacuated safely out of danger. Fortunately, Mata Nui's choices for Toa Metru did not fail him, though it was not an easy task: Teridax had summoned the Visorak hordes to invade Metru Nui, and Toa Vakama's overconfidence led the team to be mutated into half-beast Toa Hordika. However, in the end, the Toa were cured of their mutation and the Matoran were safely relocated to the tropical island above the city that the Toa had discovered and named "Mata Nui", in honor of the sleeping Great Spirit. Once there, the Toa Metru gave up their Toa power to wake the Matoran from their stasis, becoming Turaga. But their victory had come at a price - Teridax had been freed from his prison, and would certainly be back.
Another preparation made before the Cataclysm was that the Toa Mata entered Toa Canisters until the day they were needed to save the Great Spirit. When their training was completed, they entered the Codrex and were placed in their canisters. As soon as Mata Nui crashed onto the ocean planet, the canisters were launched, eventually reaching the surface of Aqua Magna, where they were supposed to float to Mata Nui. However, the canisters malfunctioned and were left drifting in the sea for a thousand years.