Sludge Accretion

Sludge is a man-made substance, often of industrial origin, which cannot be easily broken down by environmental forces. Because of this, sludge tends to accumulate in the lowest reaches of the Manifold Sky, choking out living things and making resource exploration more difficult as time goes on. 'Sludge accretions' are these accumulations of sludge and are a physical manifestation of the dangers of pollution and non-reuse of materials.

Manifestation

Being of man-made, chemical origins, sludge accretions can usually be distinguished from natural deposits of bitumen or petroleum by the fact that the sludge shows signs of having seeped down from strata overhead and will prove highly resistant to typical chemical degradation or refining processes. Sludge accretions are also toxic to plants of all types and can sometimes emit toxic vapors when heated; accretions are not, however, flammable under typical conditions.

Localization

Sludge accretions are most commonly found in underground spaces, such as caverns or aquifers, though they may be found in surface deposits in extremely polluted cube.
Sludge accretions located just under a surface will cause plants growing above them to become sickly and die; a patch of withering Penrose fescue in an otherwise verdant region could simply be blighted, but it could also mark the burial site of a toxic surprise. Sludge accretions in or near water sources create clusters of cancer, birth defects, kidney problems, and overall poor health in animal and sentient populations which rely on that water for drinking or bathing. Such tainted water may exhibit odd smells, skin irritating effects, or (worryingly for many) no signs at all of its polluted nature.
In the Northern and Southern Tesseracts, sludge accretions may also serve as environmental reservoirs of radioactive materials, which become embedded in the mass over time and, in extremely rare cases, may even achieve criticality. These glowing accretions are both a hazard and a boon - the radiation will kill unwary visitors while also breaking down the otherwise indigestible pollutants which make up its structure. Glowing accretions provide interesting observations to scientists specializing in the relatively young nuclear sciences.
Over geological time, the surfaces of cubes are eroded or moved by tectonic forces towards the commissures, where they are eventually churned into the underground magma pockets to form the stuff of new edge and vertex mountain ranges eons later. Magma is one of the few substances hot enough in the Manifold Sky setting to break down the chemicals found in sludge, converting them mostly into carbon and (relatively) harmless volatile gasses. The localized levels of activity of these geophysical processes makes sludge accretions more common in the Dorsal Tesseract and less common in the Ventral Tesseract. Still, as these processes occur over geological (rather than mortal) time scales, pockets of sludge accretion require active remediation in places which support long-term sentient inhabitation.