Lifestream

The Lifestream, also known as Spirit Energy, is an ethereal substance called Mako that flows in streams just beneath the surface of the Planet. When seen beneath the Planet, it is shown as many separate bands of green-white fluid flowing as a whole. In several places, such as Mt. Nibel and Mideel, Mako Springs shoot out of the ground, creating a local spectacle.

Nature
The Lifestream contains the essence of the Planet and the memories, emotions, and knowledge of all who have lived on it. Portions of the Lifestream are believed to be used to create new life on the Planet, and the energy of a person returns to the Planet when they die, bringing with it the emotions, memories, and knowledge they obtained during life. It is referred to many times as the life of the Planet itself, and acts as an afterlife for the conscious spirits of the inhabitants of the Planet, as well as a sort of immune system for the Planet, flowing to and "healing" scars in the Planet.

If the Planet is in danger of being utterly destroyed, the Planet summons the Omega, which destroys the remaining life on the Planet so that it returns to the Lifestream, and takes the entire Lifestream itself, launching into the cosmos to find a new planet on which life can exist.

The novella Hoshi wo Meguru Otome expands on the purpose of the Lifestream, equating it to an afterlife with a concept of Heaven and Hell. Those who led good lives join with the Lifestream and their minds fragment and join the collective, while those who were sinful remain intact and must exist with the knowledge they cannot find absolution because of their actions. Aeris Gainsborough is able to help many of these people, including Dyne and Jessie find peace and allow them to come closer to atonement.

Mako and Materia
The Shinra Electric Power Company devised a way to refine the Lifestream into a continuous electricity supplier, posting Mako Reactors around the world to extract the Lifestream from under the Planet's crust. Shinra also discovered that injecting human with pure Mako enhances their abilities, and began creating an elite warrior class called SOLDIER using Mako-enhanced humans injected with Jenova cells. However, over-exposure to Mako can cause Mako poisoning - Cloud Strife is submerged in the Lifestream for approximately a week, and while alive he is unable to form coherent thoughts and can barely speak.

Because the Lifestream is the source of life to the Planet, depleting the energy in this way causes the area surrounding Mako Reactors to wither and die. This is most evidence at Midgar, in which plants cannot grow and the surrounding area around the city is dull and barren of life. The air and water in Midgar are also highly polluted, although they may just be the pollution of the reactors themselves and not a result of the Lifestream being drained.

When Mako energy is condensed, Materia is produced. Materia allows the holder to access the Lifestream's knowledge and power and channel that power, manifesting as the phenomenon "magic". Most of the time Materia is artificially produced by forcibly condensing Mako, but at a few places, such as Mt. Nibel, natural Mako springs allow Materia to be produced naturally. It is never established in any of the Compilation of Final Fantasy VII entries if there is a difference between artificial and natural Materia besides their production.

Meteor and Holy
Two ultimate Materia were created by the Planet itself, White and Black Materia, which could both save and destroy the world respectively. Black Materia can summon Meteor, an ultra destructive force that calls a meteor to destroy the planet. The White Materia casts Holy, which brings the energy from the Lifestream to defend the Planet from any destruction. When Holy fails, Aeris directs the Lifestream to burst from the Planet and fight off Meteor from the outside, allowing Holy to succeed.

Negative Lifestream
When the Lifestream burst from the Planet, Sephiroth's spirit in it remained, and the remnants of Jenova also spread over the planet. This infected many people with the fatal disease known as Geostigma, which killed many people. Two years later, in Final Fantasy VII: Advent Children, the Remnants of Sephiroth appeared and planned a second Jenova reunion using children inflicted with Geostigma.

It is hinted in the film that the Remnants are made up of the "negative Lifestream" tainted with Jenova's cells, and they are also able to summon Shadow Creepers from this dark Lifestream. Eventually, Kadaj acquires Jenova's cells, and Sephiroth is reborn. Sephiroth at this time explains that, when those infected with Geostigma die, their tainted spirit energy will return to the Lifestream, and eventually Sephiroth will be able to use his influence over Jenova to take control of the Lifestream itself. Sephiroth summons the negative Lifestream over Midgar with his battle with Cloud Strife, but once he is defeated it fades. It is unknown what happened to the negative Lifestream afterwards.

Trivia

 * When Cloud Strife comes into his cameo in Final Fantasy Tactics, he says he remembers a "great current," possibly the Lifestream. Speculation has pointed that Cloud was whisked away for a while from the Lifestream into Ivalice by a transdimensional portal.
 * The Lifestream is the basis of "the Gaia Theory" in the film Final Fantasy: The Spirits Within. In the film, Doctor Sid speculates that all living beings hold a spiritual presence inside themselves, and when they die this spirit energy returns to a collective whole within the planet that is then used to birth new life.

Lebensstrom