"In a few short centuries, industrial civilization had spread from the western fringes of Eurasia to sprawl across the face of the planet. Plundering the soil of its riches, fouling the air, and remolding lifeforms at will, this gargantuan industrial society had already peaked a thousand years after its foundation: ahead lay abrupt and violent decline. The cities burned, welling up as clouds of poison in the war remembered as the Seven Days of Fire. The complex and sophisticated technological superstructure was lost; almost all the surface of the Earth was transformed into a sterile wasteland. Industrial civilization was never rebuilt as mankind lived on through the long twilight years..."

The world that Nausicaa was born into didn't just magically appear, it came into being through a long and intertwined series of events. The major events, known as the Daikasho, are what created the poisonous forests.

It all began when the pollution that humans bestowed onto the Earth finally reared it's ugly head. Plants began releasing a deadly gas, known as miasma, which- at it's deadliest levels- could decay a human's lungs in a matter of seconds.

These plants soon became forests, a whole big wide-spanning forest called the Sea of Corruption. The Sea of Corruption soon took over almost all of the entire Earth, destroying civilization almost entirely.

Then there is one of the biggest enigmas in the entire story; the giant insects known as the Ohmu. They are peace loving insects, vastly intelligent with wisdom perhaps beyond even that of humans. But if one of their fellow Ohmu are killed they go on a rampage, and if that killer happens to be a human, sometimes the Ohmu can chase until they starve to try to destroy that human.

One very unique aspect about the Ohmu is the spores that they drop. When they leave the forest they continue dropping their spores, which- when laid to the ground- begin to grow more deadly plants that produce miasma. So while it can be speculated that the Ohmu are the cause for the Sea of Corruption spanning the Earth, it is the humans that taunted the Ohmu forcing them to go on a rampage. When enough Ohmu's go on a rampage, that is what is known as the Daikasho.

The Ohmu are just so cool! Click here for more about the ohmu; The Ohmu you should really check it out ;)

The Story in it's entirety.

If you want to learn more about the Daikaisho, click here; Explanation of the Daikaisho's last wave