I am enormous but mostly invisible because my body is hidden under soil. when sexually mature i make spores in fruiting bodies that rise above the soil and release millions of spores. i eat by digesting dead plant material in the soil as i grow through it.

Animal Kingdom: Fungi

Organism name: Mushroom

The answer is as above since fungi are the only group (animal kingdom) that produces fruiting bodies and spores during reproductive phase. The mycelium, usually in the substrate, is the vegetative stage. Mushroom are enormous fungi whose fruiting bodies are big enough and apparent to the unaided human eye.