The Mycelium is a building exclusive to the Cymanti tribe that can connect to cities and other Mycelium through field, Forest, and Algae tiles within a 3 tile radius. You can only build one Mycelium per city. Mycelium can only be built on field and forest tiles. Mycelium heals any friendly unit that end their turn on or next to them by 4 hp and also cures poison. If a city with a mycelium in it is captured by a non-Cymanti tribe, the mycelium will start healing that tribe's unit instead of the Cymanti tribe's units.
Compared to Roads, Mycelium is on average a much more cost-effective way of connecting cities on contiguous landmasses.
Mycelium levels up depending on how many different trade routes lead into it.