Thunderhead was a sporemind that existed in the early years of the ten thousandth century. It appeared dramatically above the city of Ar Belex within the Myg Rainforest in Y9909*, a dense mass of spores with the appearance of a cloud before a storm, the source of its name. It soon consumed the residents of Ar Belex, save for a few who were upwind and far enough away in the outlying farms, after which it grew mycelial networks following the city streets and canals and directed the spore-carried bodies of the inhabitants along them. Thunderhead's obsession with the patterns of the city led to it failing to expand, and by Y9915* it had dissipated and disappeared.