Makuta 's servant Kohra strategically placed and detonated large charges of valmai powder within Onu-Koro 's Great Mine. The event, which came to be colloquially known as the 'Great Mine Bombing', caused many casualties as well as destroyed and flooded portions of the mine. Evidence left behind however pointed to a Matoran perpetrator.
Background
Kohra's purpose as a servant of the Makuta primarily consisted of sabotaging the three virtues of unity, duty, and destiny. If she could succeed in making the Matoran distrust each other and neglect their duties and a belief in destiny it would help the Makuta greatly. After the poisoning of Po-Koro 's well, Kohra, accompanied by Toa Clo'On traveled to Onu-Koro. The Toa of Stone had encountered her as she was about to leave Po-Koro, only agreeing to let him come along as to not raise suspicion. Once in Onu-Koro, the Vortixx had to lose the Toa so she would not be exposed as a servant of the Makuta.
Once she had lost the Toa, the Vortixx obtained a forged copy of the mining-plans of the great mine from one of the Makuta's agents. She also received the name of a being in Le-Wahi, who would supply her with the explosives necessary for her plan.
Following her masters instructions she traveled to the village of air and found the being in question: Draeverian Joskiir, a blind Toa of Sound. After being threatened with death he agreed to help her without knowing the ultimate goal and they traveled to Ko-Koro, where he gave Kohra the necessary items to create the explosives: Valmai-powder and retardant charges to set the powder off.
With Draeverian in tow and everything ready, Kohra traveled back to Onu-Koro. Back in the underground village, Kohra and Draeverian checked into an inn near the markets. While the blind Toa continued to try and figure out Kohra's plans, she carefully studied the plans of the great mine.
The Bombing
Once she was sure how to proceed and where to plant the charges, she quietly slipped out of the inn while the Toa was out of the room and made her way to the great mine. Distracting the Ussalry-guards at its entrance, the Vortixx gained access by entering the mine through the ventilation-shafts.
Once inside, she snuck into the room of the foremen, where the plans were kept, and replaced the actual plans for mining-operations for the next shift with the fake copy, leading the workers to work in tunnels that had been abandoned due to danger of flooding.
Next, she waited for the change of shifts to access the lower levels of the mine, where she planted the explosive charges at strategic weak-points. When the new workers began their shift, she killed one that had been ordered to scout ahead to scout out a tunnel and used his blood to write a message on the wall, before leaving the mine again and Onu-Koro itself shortly afterwards.
Shortly after the change of shifts in the great mine, a dozen explosive charges detonated across the eastern tunnels of the lower three levels of the great mine. The strategic placement of the charges caused sea-water to flood into these levels at multiple points.
Several miners were either caught in the explosions or drowned in the subsequent flooding of their tunnels. The extensive damage to the lowest three levels caused the water to rise at an alarming speed, until the affected part of the great mine was completely submerged.
Aftermath
A team of Onu-Matoran miners working on the lowest level of the mine drowned in the flooding of the lower levels, as they were trapped in a damaged elevator at the time. The 1st, 2nd and 3rd lower levels of the mine suffered extensive damage from the explosions themselves, including access-ways like elevators and ramps or ladders.
After Makuta's defeat Akiri Nuparu led a rebuilding effort. The Great Mines of Onu-Koro are as they once were and running at full capacity.