Fossil

Fossil is a rare underground skeletal feature that generates naturally in Minecraft's desert, swamp, and mangrove swamp biomes. Constructed from bone blocks interspersed with either coal ore or diamond ore, a fossil takes one of eight possible shapes — four skull variants and four spine variants — each selected with equal probability whenever one spawns.

Each chunk makes two separate generation attempts: one within the Y range of 0 to 320, and another between Y -63 and -8, with each attempt carrying only a 1-in-64 chance of success. When one does form, it is built in two sequential passes. The first pass places the bone block skeleton at a structural integrity of 0.9, leaving roughly 10% of positions unfilled. The second pass then applies an ore overlay at an integrity of 0.1, replacing around 10% of the remaining blocks with ore — coal ore in fossils above Y=0, and diamond ore or deepslate diamond ore in those below Y=-8. Because the two passes rarely void and fill the exact same spots, the resulting structure is typically an irregular mixture of bone blocks and embedded ores.

Unlike most underground structures, the structure is classified as a world feature rather than a true structure, which means it will generate even when the 'Generate Structures' world option is disabled. Skull shapes range from 32 to 86 blocks in total size, while spine shapes range from 37 to 121 blocks, making the fossil one of the more variable naturally generated formations in the game.