The Underdark
The Underdark are the basic description used for regions under the earth that can be explored. It can hold a numerous amount of treasure, lost civilisations, and dangers. Some of these can be found in both, while others are either near the surface, or deeper down. It is split into the Upperdark and the Lowerdark.
The Upperdark
The Upperdark usually consists of caves & caverns, gullies, mines and rents in the earth that can lead an explorer down into the upper layers of bedrock. These are caused in many instances by natural phenomena, such as glaciers, water in the form of rivers for example, sinkholes, volcanic activity, or earthquakes. in some cases, mining activities, the use of magic, certain underground animals (especially burrowers), or elemental creatures can cause tunnels & caves to form, and then weathered by nature over time.
Many animals can and do make their way into these caves and adapt over many generations to live down in them. Some intelligent beings enter to mine or even live down in these. This is true if the latter is driven out of a region and has no other place to go. Some go down to hunt for precious metal ores or gems and never come back, and others use them for raids on the surface.
Water can make its way down here, especially if a stream, river, or lake exists above. A few are below glaciers that can cause the rock below it to buckle and open up., as well as run off from friction against the rock. Sometimes plants and similar organisms make it down, and microbes are found in many of these caves, along with fungal growths, lichens and the like. Some of these are bioluminescent and give off a lambent tight in the surrounding area. A rare few animals do too, and most use other senses, or have their eyes specially developed to see in low or no light. Not every cave or complex has anything larger than a bacteria or mould living in these conditions though, usually using some form of anaerobic respiration and feeding on minerals in the rock. Nearly every higher lifeform here feeds on something else living in the caves. The biggest concern for most organisms is changes to any air in the environment. Sometimes noxious gasses exist or build up making it impossible to breathe. This is especially true in the case where volcanic activity in some systems, and also if the local organisms don't need gaseous oxygen to breathe and survive. These caves & tunnels can extend miles underground going up to 10 or more miles down. Nearly everything here is natural, with a few artificial environments created by creatures to mine, or have settlements in, especially Dwarves, and Deep Gnomes. Not all these systems are interlinked. Many are in different regions with only the Lowerdark linking them.
The Lowerdark
Below the Upperdark is the Lowerdark. It is a mass if caves, caverns and rents in the rock beds, and much more dangerous. Closer to the core of the planet, it can be warmer, and with less water in many regions. The caverns here can be huge, and the regions closer to the Upperdark has more breathable air, potable water, and plants, animals & even sentient species of which a few have cities built in these chambers. Some of the sentient creatures have expanded the caverns, with building more for space to grow, mining, larger cities and collecting water, to name but a few. A few are friendly like some Dwarven clans, and Deep Gnome clans, though many are evil such as the Black Elves and Duergar. Groups like this rarely stray too far down, mainly because of instability in some regions, volcanic activity deep down and rumours of ancient evils lurking in the darkest corners & lowest levels. Many of these settlement are found between 10 and 15 miles down at the deepest. Anything below this is just too dangerous. There are hints by many of the intelligent creatures here that the Lowerdark stretches round the whole world, though there is no way to actually prove it at present.
Many of the organisms found at higher levels can be found down here too though there are plenty of other even more bizarre and in some cases ancient beings too, and most are highly dangerous if not outright hostile and evil. Rumour has it ancient remnants of lost civilisations exist here, many home to the twisted nightmares that the original inhabitants have become. Others state that the region is populated by aeons old evil in the form of deities and their minions, called down to earth by their kin, or put there never to bother the younger races that inhabit the surface. In many cases there are the rumours of ancient magic still there that can be tapped, and millennial old caches of powerful magic items. In either case these deities and items are spoken of in hushed tones in many overground civilisations where they whisper that these call out to the surface world to escape the Underdark, and be able to grow strong again. Also, they look to dominate the world and bring the newer races to heel under their leadership.
Very few adventurers and explorers have ventured to any great depth in the Lowerdark, and those that have, even less has returned, at least sane, if not crippled outright, spiritually, physically and / or mentally by their experiences. Some take up with the Dwarves & Deep Gnomes in some capacity, while others end up captured by the Black Elves and never seen again, while a few completely vanish without a trace. The fate of these unfortunates are best only guessed at.
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