The swampland of Mirrodin, called the Mephidross or "The Dross," is a knee-deep sea of foul water covered over by a scum of black ichor. It is a toxic wasteland of ooze, filth, and corrosion. Here, a dread vapor known as necrogen slowly turns the unfortunate into the Nim, a race of zombie-like creatures well suited in hunting down the living.
Like the Quicksilver Sea, the Dross has spires as well -- but not like the spires constructed by the Vedalken. These are ominous, dark obelisks whose droning hum spreads throughout the Mephidross. In fact the Dross's most immediately striking feature is this constant thrumming sound, suggesting the idea that the swamp may be alive, or perhaps powered by a dark energy.
From a distance, the sky above the Dross looks like ink swirling in water. As you approach it, the ground begins to soften, leaving spongy footprints in the land. But the most striking feature of the Dross is the sound it makes. The whole swamp hums with a low, dissonant chord. In some places, it sounds like a distant generator. In others, it's deafening. Some report hearing whispers beneath the constant thrum. The sound comes from the chimneys of the Dross, ominous structures that "grow" seemingly at random throughout Mephidross. They range from ten to three hundred feet high. All emit the sickly green gas called necrogen that slowly saps the life from creatures, turning them into nim.
These spires are like gunmetal chimneys that radiate a magical energy over all who dwell there. This energy flows in the form of necrogen, an noxious gas that twists and transforms those it touches. The undead-like Nim are one result of the corrupting power of necrogen.
At the center of the Mephidross lies Ish Sah, the Vault of Whispers.
The human residents of the Dross, the Moriok, try to remain hidden and eke out an existence scavenging among the Mephidross' many scrapheaps.