The Underdown is an enormous network of underground tunnels which spans the width of the Oceanykan Federation. Its name is a pun on Australia's nickname as "The Land Down Under", having such an expansive realm even below itself. Nobody is sure of its origins, true size or degree of habitation. However, one thing is certain; though there are many places in Oceanyka where life shouldn't thrive and yet does, perhaps it is the Underdown that takes the crown for the most hostile environment of all, even beyond the hellish Tasmanian Highlands. Because there is no light, life below the surface relies on fungal bioaccumulative radiothrophy (FBR). This unique process involves vast mycelium networks which drill deep into the rock, extracting ore with fissile material in it. The mycelium transports this material into a processing node where it undergoes radioisotope separation. The resulting radioactive material is then distributed amongst the fungal colony, to be deposited in thin rod-like organs which generate energy. In summary, each fungal colony is a biological nuclear reactor. They also tend to be strongly bioluminescent.
It goes without saying that the Underdown is extremely radioactive. The very few animal species which have adapted to life consuming fungi in this realm are always horribly scarred, mutated and cancer riddled. Another factor is the greater concentration of Warpstone as a result of FBR metabolism, which some scientists believe is responsible for otherwise unbelievable stories of strange appearances and cryptids deep into the Earth.
Almost all natural caves in Oceanyka lead to the Underdown, as do many ancient and modern mines.