Minecraft wiki entry for Ancient_Debris, associated text: Ancient debris generates in the Nether in the form of blobs. Up to two blobs may generate per chunk: one blob of 1–4 ancient debris attempts to generate with a periodic normal distribution (similarly to lapis lazuli ore) from levels 8 to 22, and an additional blob of 0–2 ancient debris attempts to generate randomly from levels 8 to 119. On average, Y-level 15 has the most ancient debris.