It is entirely possible that underground there are species of bacteria and fungi unknown to science that could provide a source of new medicines.
Most fungi will have a preference for the food source, those that live on wood will prefer a certain type of tree and this can be used to help identify it. Unfortunately wood found underground is usually rotten and difficult to identify.
Some of the typical fungi seen underground on wood are:
Honey Fungus Armellia mellea - this is a parasite of conifers and broad leaved trees, and often kills its host. It produces long, black, bootlace like rhizomorphs between the bark and the wood. The mycelium of Honey fungus is also luminescent.
Candle snuff (Xylaria hypoxylon) and Dead Mans Fingers (Xylaria polymorpha) are found on deciduous wood, often beech.