Earth Crisis

The Earth Crisis is a historical event of the Maverick Wars, in which the Earth itself is endangered by the fall of space colony Eurasia. The world is saved by the Maverick Hunters, but the resulting damage to the surface forces humanity underground for an indeterminate period. It would take a long time for the Earth to recover, with lingering consequences.

Prelude
Several months after the Great Repliforce War, the Maverick Sigma plots the fall of Eurasia in conjunction with an old man in a scheme to awaken Zero's true self: the plan was to spread the Sigma Virus across both the Earth and Eurasia, then combine the two strains to create a new virus in order to infect Zero and make him destroy Mega Man X.

After turning Eurasia into a virus colony, Sigma employs the mysterious Dynamo to drop it from orbit while Sigma himself tricks X and Zero into spreading the virus across the Earth. This results on an unprecedented Maverick epidemic, with countless Reploids infected on a global scale. With only 16 hours before Eurasia crashes on Earth, the Maverick Hunters first attempt to blast the colony with the Enigma cannon. When this plan fails, the Maverick Hunters resort to a more desperate gambit: strike Eurasia with a space shuttle. Zero volunteers to pilot the shuttle, taking the Earth's fate on his hands.

Epilogue
The attack is successful: Eurasia is destroyed, and Zero returns safely to Earth. However, the resulting explosion causes major damage to the planet. This event also creates the powerful Zero Virus, and even produces a temporary rift between the Earth and Cyberspace. Sigma himself is defeated, but Zero goes missing in action.

As a result of Sigma's actions, many Hunters are lost during the crisis. In addition, the pollution makes the surface inhospitable, driving humanity underground. The troposphere suffers catastrophic damage, the seas become unfit for life and the remnants of Eurasia create a huge crater on the Earth.

Incidents
With the humans underground, the Reploids begin the recovery efforts with great difficulty. Three weeks later, Gate, an outcast scientist infected by the virus attempts to take advantage of the Earth's weakened state, by taking over the Reploids with a virus created from Zero's DNA called the Nightmare. This scheme is stopped by X and a returning Zero, who together defeat Gate and a returning Sigma.

Recovery
Thanks to the Reploids' work, even the most devastated regions are set on the road to recovery. However, the continuation of the Maverick Wars hinders reconstruction.

After the end of the Elf Wars, the Four Guardians of Neo Arcadia use their abilities to gradually restore the Earth. They also act as the controls for a weather controlling station called Weather Orbit before being recalled to fight the Resistance.

It is later discovered to much surprise that nature is returning in Eurasia's crash site, providing refuge for a human caravan attempting to escape the tyranny of Neo Arcadia's new leader Dr. Weil. Area Zero is saved from Dr. Weil's Operation Ragnarok by Zero, at the cost of his life.

The seas are restored after the war, although the Earth's repopulation pollutes them again.

Trivia

 * The name "Earth Crisis" is coined in Mega Man X6.