A blessed energy resource turned corruptor of power-thirsty humans, mako is the very life and blood of the planet Gaia. The abuse of it by the humans of the planet prompt the events of the Jenovian war and beyond.
Materia
Where large amounts of mako pocket in the earth, crystaline forms of it grow. Such areas are rare; one of the only known areas where this occurs is in the mountains of Nibelheim, where a spring of mako crystals exists. From the crystals, materia is created through a refining process. Being originally the spiritual force of the planet gives materia their special powers.
Midgar
Midgar was the first city to fully utilise the mako energy resource by Reeve's own design. As architect of the city, Reeve oversaw the development of the Mako Reactors that would eventually suck so much of the natural resource from the planet that it would be provoked to use the most deadly means at its disposal to cure itself of the parasitic force.
Reeve was, at the time, unaware of the effect of harvesting mako -- and so much of it -- from the depths of the planet. He saw only a means to end a problem -- the problem of a lack of a sufficient energy resource -- and mako energy solved that problem. With the bountiful amounts of mako collected, the city of Midgar was given ample amounts of electrical power, which is reflected through the generous use of electricity and electronics throughout the city.
Post-Meteorfall, the city of Midgar has crumbled to near nothingness. Although plans were made to rebuild it, none came to fruition due to the abscene amounts of mako radiation leaking from the destroyed Mako Reactors.
Poison
Like other energy sources that exist in our world, the mako of Gaia is can be a dangerous substance. In small amounts, mako can have a beneficial effect, particularly when refined into materia. But large, direct exposures to mako can have disasterous effects, such as the delirious and near-comatose state Cloud was thrown into after coming in contact with a large amount of mako.
Unlike other poisons, which may cause delirium as well as physical damage, the effects of mako poison reflect the spiritual origins of mako and in turn have a psychological, rather than physical, effect. A prime example is Cloud, whose memories became increasingly distorted almost to the point of insanity, had it not been for Tifa helping him sort them out in the Lifestream. Cloud has the mako eyes associated with exposure to high amounts of mako.
Geostigma
A disease that wittles away at the body and soul, Geostigma came to be after Jenova's cells were released into the Lifestream at Sephiroth's defeat. Thousands are infected by the disease by way of the Lifestream, which burst from the earth too close to the cities of Kalm and Midgar. Although Aerith is able to heal many of those suffering from Geostigma, cases still exist in the present day.
