The Underworld is a network of caves, caverns and tunnels that stretch across huge distances underneath the surface of Kaelaross. The exact size is unknown, but there are connections to the Underworld all over Toutus, from the village of Pordril in the far north to the Kaynor Islands in the west to the Steelpick Clan of the Dwarves of the Ten Peaks.
These are the known connections to the surface - there may be hundreds as yet undiscovered.
The Underworld is mostly natural caves, particularly limestone with its distinctive stalactites, stalagmites and columns. Lava formations are also found in the network. Creation or modification of new spaces either by intelligent creatures such as goblins or mindless burrowing things such as purple worms also contribute to a significant proportion - perhaps 10-15% in some densely populated areas of the Underworld will be carved by creatures rather than natural processes.
The underworld is not just a flat area - it is a three-dimensional environment, both on a small scale and on a grand scale. Sloping tunnels, chasms, sinkholes and the like mean that depth is as important as horizontal distance, particularly in relation to the surface. Mappers need to take this into account when exploring - gently sloping tunnels crisscrossing over each other can fool even the most careful cartographer. Nobody knows how deep the Underworld really goes, but some isolated clans of dwarves, who could only talk to explorers from the surface by means of magical spells, have reported that they have visited caverns 3 miles below their own homes, and their homes are at least 2 miles from the surface. Of course, accurate measurement is difficult without magical divination, but these dwarves have an uncanny perception of where they are underground.
There are certain habitats within the Underworld that will attract different creatures.
Underworld seas - These large bodies of water may be fresh or salty, and may be of any size though most folk agree that an underworld lake becomes an underworld sea when it stretches more than a mile in width and length. Such underworld seas can contain many different creatures - some may be variations of creatures from the surface oceans (albino octopi and blind fish), while others are completely alien and bizarre. On the shores of some of these underworld seas there are seaweedlike fungi that support other life forms. There are even stories from the deep dwarves of islands in the middle of underworld seas with their own creatures and cultures.
Fungal Forests are often found in large caverns with adequate moisture, possibly a stream or a river flowing through. Here the fungi are not restricted by cavern height, and can grow to the size of surface trees. These fungal forests can support a rich variety of living creatures that feed off the fungi. These in turn provide food for many predators, many of which are dangerous to human explorers. Intelligent living creatures often live near to Fungal Forests as they too need food.
Volcanic tunnels and chambers technically need not be active, but most folk think of golden-hot magma in pools and streams, of sulphuric vents and rocks too hot to walk on in boots let alone touch. These volcanic areas are generally inhospitable to organic life. Creatures from other planes, and those with a magical connection to fire will enjoy such places. Human and dwarf explorers with magical protection against fire have encountered fire salamanders, clans of fire giants, red dragons and even an outpost of efreet in an obsidian citadel on an island in the middle of a lava lake.
Crystal Gardens are weird and so far inexplicable. They are areas where crystals have grown to unusually large sizes and strange shapes. Nearly all the time the crystals are of relatively common minerals - star bursts of quartz the size of barrels, gypsum "flowers" bigger than cabbages, columns of malachite towering for several yards upwards. It is, of course, the adventurer's dream to hit the jackpot and find a massive precious gem though if anyone has done so they have not lived to tell the tale. The inhabitants of these crystal gardens are peculiar - sometimes organic (fungi sometimes grow inbetween the crystals), sometimes elemental and rarely a strange fusion of the two types.
Chaos-Tainted regions are considerably worse than other areas. Such regions are populated by chaotic monsters, and there may be a chaos portal in the vicinity, linking the location from the Underworld to one of the Planes of Chaos. Some chaos-tainted regions are inhospitable to normal life - not even fungi will grow. In these dead areas undead of various types can be found. In other chaos-tainted regions, strange magical effects warp the very surrounding rock and turn underworld creatures into monstrous mockeries of their former selves.
Subterranean Cities are areas created by (relatively) intelligent beings, the four most common races being goblins, dwarves, snakemen and troglodytes. They may be conventional buildings constructed within a large enough cavern, or they may be a network of rooms, tunnels and chambers dug straight into the rock. Sometimes these subterranean cities are still inhabited by those who built them but often they have either been abandoned or changed hands. Current subterranean cities will have basic supplies available - food and water are the most important. Abandoned cities are usually not entirely devoid of creatures, but there may also be treasure left behind by the previous inhabitants.