Minecraft wiki entry for Mycelium, associated text: Naturally generated mycelium in a mushroom fields biome.
An another view of naturally generated mycelium in a mushroom fields biome.
Mycelium as shown with snow on top. The mycelium still produces spores.
Mycelium as seen spreading in dirt and destroying planted sugar cane. This is no longer possible as of 1.17, as sugar cane can now be planted on mycelium.