The Underdark

The Underdark of Eurra was a vast, winding network of tunnels and caverns that ranged under much of the surface of Eurra. It could be accessed through countless points in the world and went many miles deep in some places. The Underdark was home to many evils monsters and aberrations that would prey upon unlucky visitors that lost their way. While it is believed that all the known Underdark realms are connected and traverse the planet, no explorer or adventurer has ever come back with proof of it to be the case. With that in mind, some explorers and experts on the Underdark focus their study on particular areas that have been ventured into the most.

Geography

The Crystal Underdark

A portion of the Underdark that seemed to have access points in several places in southern Vyrania that was surprisingly as beautiful as it was dangerous, the Crystal Underdark was largely situated below the lands of the Dragonborn Emirate. This realm was known to be very hot and many surface races, other than the Dragonborn, usually found this realm uncomfortable for extended stays. The dry heat could quickly send an unprepared adventurer into dehydration or hyperthermia. However, this heat was also a boon to the abundant minerals below the surface that would react to it by forming the crystals that gave this place its name. Tunnels were long and lined with natural obelisks of different colored minerals and entire cavern chambers could be geodes hundreds of feet across.

The Everspring

Unlike the majority of caves that eventually breach into the Underdark, this place is surprisingly open. Located somewhere within the borders of the Elven Leagues, this place is a huge cenote going hundreds of feet from the surface down to a large underground lake.

The Jordish Underdark

The Dwarven homeland of Jord was known to have a vast Underdark cavern system and was one of the earliest recorded due to the delving nature of the continent's denizens. It ranged under both Inner and Outer Jord. Additionally, due to the geothermal and volcanic activity throughout Jord, the tunnels of that expanse of Underdark were known to change periodically. Flowing ocean water and snow melt from the surface formed rivers that also helped shape much of this domain. The shifting paths were made all the more dangerous by the fact that many creatures making their home there were capable of creating illusions that would alter the appearance of the tunnels, only to lead to them a dead end or pitfall to become easy prey. Civilized Kobold tribes were known to live in large tribes in the upper reaches of the Underdark, while Duergar resided further down. There were even scattered reports of Drow being seen within the same areas.
The Jordish Underdark even played a role in several myths of the Dwarves, most notably as the den of the dragon Morhid.
Most recently, the Bad Luck Blitzkrieg adventuring party traversed a portion of this realm during their quest to escape the prison of Inner Jord in the spring of 1436 UE.

Ecosystem

All the known Underdark realms had unique features, but they also largely shared certain commonalities. Many of the upper cavern systems were usually a stable, constant temperature comfortable to most races that ventured into them. The deeper into Eurra a traveler went, however, the more likely they were to encounter magma flows and corresponding heat. The climate could also vary by the amounts of water that would work its way down below the surface. The moisture would sometimes allow the growth of bioluminescent plants. In other stretches of caverns, oddly glowing crystals would dispel many an adventurer's thought that their explorations would be entirely in pitch black. And this is to say nothing of the oftentimes horrid beings living below capable of magically creating ambient lights to drawn in the unsuspecting.
With that said, moving to the inhabitants of the Underdark, it must be said that the majority of interactions observed by careful mages and scouts were based in predator/prey relationships. Even among the more civilized creatures one may encounter, it should almost never be taken at face value that you are a guest. Your presence as an outsider merely makes you an easier potential victim. 
Drow  To most, these subterranean elves are a race of demon-worshiping marauders dwelling in the subterranean depths of the Underdark. Drow have been known to emerge on the surface only on the blackest nights to pillage and slaughter the surface dwellers they despise. Their society is depraved and preoccupied with sanctioned murder from the dark gods they follow and are likely to take advantage of any opportunity to make worthy sacrifices to those gods. This is not always the case, but an assumption an outsider should make for safety's sake. 
Duergar The Duergar offshoots of Dwarves are highly territorial and just as martial as their surface cousins. They make great fortress halls and cities, taking advantage of the largest caves to make the best defense against monstrous creatures. Duergar make war against their Dwarven kin and all other subterranean races whenever they have encounters. They will forge alliances when it is convenient, most commonly with Drow, then break those alliances when they have nothing more to gain. They take and hold slaves to toil in the Underdark, regarding them as free labor and crude currency.
Illithids Since the fall of their empires in the far-flung past, illithid collectives on the Material Plane have resided in the Underdark. More commonly known as mind flayers, they are the scourge of sentient creatures. These secretive beings seek to harvest entire races for their own twisted ends. Four tentacles snake from their octopus-like heads, flexing in hungry anticipation when sentient creatures come near. Linked by a collective consciousness, they hatch plots as far-reaching and evil as their fathomless minds can conceive.
Kobolds Kobolds may be the race that makes the most frequent contact with the surface. These small reptilians live in tribes and are known to serve dragons, placing their settlements inside or adjacent to their dens.

Location under

Eurra

Related Reports (Secondary)

• 13th Session- The Jord Escape