"The Sea of Corruption was the new world... An ecological system born in the polluted wastelands created by civilizations long past. Only the great insects could live among the giant fungi and the miasma they exhaled, and so the earth was slowly submerging beneath that decaying sea..." [1]
The Sea of Corruption is a vast poisonous forest that constantly threatens the thin areas where humans can live. The forest is filled with a deadly miasma that chokes all human and animal life. Only those wearing a Shohki Mask or the equivalent can hope to survive in this deadly place.
The Sea has several different layers. The "trees" release spores in the afternoon causing updrafts in the air [2].
Insects like the Ohmu are unaffected by the miasma and when they travel out of the Sea of Corruption, they bring deadly spores that can easily take root in clean lands, which in turn spreads the Sea further. Most communities close to the Sea of Corruption have elaborate methods, including using Fire Wands, to prevent the spread of these deadly spores.
The Sea of Corruption is also colloquially called The Rotwood [3] by the inhabitants of the Valley of the Wind.
Strata
The Sea of Corruption has several layers or strata [4]. The topmost and most poisonous is the Active Layer. The middle layer is still poisonous and is known as the Petrified Layer. The bottom is known as the Vaults. The air is clear and there is no miasma there [5]. The strata is a centuries-long process to turn the poisons of the miasma into "harmless, stable elements...reduced to tiny crystals, too small for the eye to see" and clean the world [6].
Plants
The plants in the Sea of Corruption are made up of various fungi that release deadly spores into a Miasma.
The plants scent food, like dead bodies, and release spores to grow the Sea of Corruption, eventually covering the food with spores [7]. The spores take root in the corpses and the deadly fungus grows.
People
There are two groups that interact with the Sea of Corruption on a near-daily basis: the Forest People and Worm Handlers.
Growth
The Sea of Corruption is spreading. When insects get angry, they go on rampages and inadvertently expose clean land to deadly spores.
Yupa reports three countries were swallowed up by the Sea in the South in the past several years and argues that the continent will be covered by the Sea within a century [8].
References
a See Page Conversions Between Editions to convert Deluxe Edition volume 1 (DE1) to other editions