Final Fantasy VII
Mako Reactors are effectively power planets created by ShinRa to provide power to Midgar, amongst other things, and they are first encountered right near the start of Final Fantasy VII. At this stage in the game, Cloud Strife is on a mission to destroy one of them with Barret Wallace.
While they provide lots of power, they also seriously harm the planet. Many have noticed that the life is sucked from regions that contain said reactors and this is because it uses the Lifestream in surrounding areas to create its energy.
How Many Mako Reactors Are In Midgar?
There are eight different Mako Reactors which surround Midgar. These are President ShinRa's pride and joy, but he hopes to one day make Neo-Midgar in the Promised Land which will eclipse Midgar's energy potential.
During the early stages of Final Fantasy VII, AVALANCHE undertake missions to try and destroy the Mako Reactors. They succeed in blowing up both Mako Reactor No. 1 and Mako Reactor No. 5, but they aren't out of commission for long.
Later on, they are used to help power the Sister Ray, after it is moved to Midgar from Junon. All eight are used to feed Mako energy directly into the cannon and this helps to destroy the shield around the Northern Crater and to defeat Diamond Weapon.
Fort Condor Mako Reactor
The Mako Reactor at Fort Condor actually houses one of the huge materia that ShinRa needs. However, a large Condor lives on top of the reactor and this hinders ShinRa's plans to retrieve it.
ShinRa attempt to remove the condor and after they defeat the local rebels all hope seems lost for the condor. It eventually dies, despite the player group managing to ward off ShinRa forces, but its egg hatches meaning the condor will live on.
Mt. Nibel (Nibelheim) Mako Reactor
This is probably the most famous of the Mako Reactors due to the events that take place inside and it was the first Mako Reactor ever built by ShinRa. Numerous experiments are performed by ShinRa using Mako energy at this reactor and it also housed Jenova at one point.
The mission that Sephiroth and Zack undertake in the Nibelheim Mako Reactor is pivotal to the Final Fantasy VII story as it is here that Sephiroth learns about his past. He also manages to steal Jenova's head before Jenova is moved back to the ShinRa Headquarters.
Junon Underwater Reactor
Despite few residents knowing about it, there is actually a Mako Reactor in Junon which is found underwater. The reactor can be accessed either by a long elevator from Junon, or via submarine.
It is from here that The Turks, on behalf of ShinRa, try to escape with one of the huge materia. Their submarine ends up being caught and the huge materia is stolen from their grasp.
Gongaga Mako Reactor
A Mako Reactor was installed in Gongaga, the hometown of Zack Fair, due to its remote nature. However, something terrible happened and there was a huge explosion at the reactor.
This explosion destroyed a chunk of Gongaga and left a noticable blot in an otherwise serene landscape surrounding the region. A lot of villagers were also killed when the reactor exploded.
Much controversy surrounded the creation of the Corel Mako Reactor, as while many of the town's residents were strongly opposed to the idea, Barret Wallace was strongly in favour of the idea. He managed to convince the townsfolk to sell their mine to ShinRa, so that they could build a Mako Reactor.
Things were going absolutely fine until one day there was an explosion at the reactor. ShinRa held the town responsible and carried out mass genocide, killing many of the people in the town, including Barret's wife. The surviving members placed the blamed solely on Barret and were forced to set up a new town called North Corel.
ShinRa were able to repair the Mako Reactor and used it to produce a huge materia, which is then transported by train to North Corel before it is stolen.
Before Crisis: Final Fantasy VII
After Barret had given the go-ahead for ShinRa to build the Mako Reactor in Corel, AVALANCHE attempted to disrupt its construction. During this time, Barret helped ShinRa to fight off AVALANCHE.
However, after it was completed, AVALANCHE were successful in their attempts to bomb the reactor and instead of blaming AVALANCHE for the attack, ShinRa decided to hold the town responsible. Barret didn't even know that AVALANCHE were the ones behind the bombing, but he might have thought differently about them had he known.
Dirge of Cerberus: Final Fantasy VII
With the existance of DeepGround disclosed, Mako Reactor 0 is also revealed. It is a secret Mako Reactor below the ShinRa building and is used by the Tsivets as their base of operations. Weiss' throne is situated at the hub of Mako Reactor 0, awaiting the rise of Omega Weapon.
Harnessing The Power of Omega Weapon
While under the control of Professor Hojo, Weiss hatches a plan to harness the power of Omega Weapon through the disused Mako Reactors in Midgar. Once Omega is eventually summoned, it draws energy from the Mako Reactors, which causes it to become incredibly strong.
However, the former members of AVALANCHE manage to destroy the reactors, giving Vincent Valentine the opportunity to defeat Omega.
Crisis Core: Final Fantasy VII
It is disclosed in Crisis Core that a reactor was planned to be built in Modeoheim. However, ShinRa's plans were for some reason cancelled while the reactor was being built and it was never completed.
Many of the residents left the town, including most of the scientists, but activity in the area was rekindled by Genesis as he uses it as a base for his operations.
The Day Sephiroth Changed
Zack and Sephiroth are sent to investigate the increase of monsters in the Nibelheim region and the Mt. Nibel Mako Reactor is the suspected cause. As the two venture inside the reactor, they find some of the hideous experiments that had been conducted.
While they're investigating, Genesis appears and informs Sephiroth about his existance. It is this pivotal moment which shapes the future story in the Final Fantasy VII franchise.