Plastic
Plastic has become one of the most common traces of civilization still found in the Fifth World. Civilization produced billions of tons of the material before its collapse. The enormous ocean gyres where it once accumulated broke down quickly — it only takes a few years for most plastics to break down under sunlight — but the beads of translucent sand that remained will last for centuries to come. That sand eventually began to wash up on beaches along the new coastlines carved by melting ice caps and rising seas, creating eerily translucent beaches.
Where extreme heat came to act on plastic waste, as with fires or lava flows, it produces plastiglomerate: a rock made of sediment and other natural debris fused in hardened, molten plastic.
With such an abundant resource, some bacteria took advantage of the new niche — sometimes aided by the genetic legacies of ancient experiments to produce strains that would consume plastic. When some of these bacteria began to grow in the digestive tracts of ruminants, they began seeking out and eating plastic.