The Void

"Hear a lass weeping near void yer fine, but if laughter catches yer ear, ya have 'til nine." - An excerpt from a collection of folk tales of the underdark.

Located in the eastern depths of the Underdark, this blue hole stretches across a large underground lake. It's located in the center and takes up around sixty percent of the lake's mass. Scientists are unsure of how old it is or just how deep the hole goes. Swimming in the lake is unadvised as even the normal waters are deceivingly deep. This doesn't stop daring teens and young adults from attempted explorations. There is usually a member of Agi's Scales posted to keep watch and make sure no one gets hurt.

Known Species

Despite the difficult terrain, scientists have managed to observe at least a small percentage of species that make up the lake's ecosystem. As technology improves, they are able to send remote controlled drones into the deeper portions. It is common to find blind or light sensitive creatures, especially in the deeper sections. Bioluminescent algae covers large portions of the rocks and other structures. The small amount of plant life typically grows in the upper levels where there is access to light.
Researchers have discovered slow moving bony fish, numerous crustaceans, a few amphibians, many invertebrates, and a vast amount of microorganisms. Studies provide more insight into how other native species to the Underdark are able to thrive in low light conditions. Discoveries like these also help scientists understand other ecosystems like the deep ocean and other cave systems. So far they've managed to reach the normal lake bottom. The blue hole itself has yet to be fully explored. Its full depth is still unknown. 

Water Composition

Observing fish and measuring depths may be hard, but analyzing the lake's water was much easier for scientists. The lake is composed of mineral heavy freshwater. These components are leeched from the surrounding rock and soil. Despite all the extra nutrients, it is still a struggle for the wildlife due to a low oxygen content.  This reflects in some of the adaptive evolution researchers noted while observing its different species. If filtered to remove large particles, the water is drinkable and harmless. 

Folklore 

Since the void has been around from the beginning, many cautionary tales and legends arose. Most were strewn of parental worries, hoping to curb their children from drowning in the unknown depths. Some tales are imaginative musings of what secrets lay at the bottom or why its there in the first place. The rest are tales told by the fire after fateful encounters with possible entities along the shore. 
The Weeping Lass  is the most infamous legend. It originated around 207 after a drunk camper came rushing back to town. No one knows exactly how it went, but the incident left residents rattled. An unknown listener wrote down his rambling to the best of their ability. Others took heed and passed this information down the line. Over time it spread across the entire Underdark. Some details may have been lost. Now parents read this tale among others at bedtime while campers whisper it across the fire. 

"What I saw was real, I tell ya! My mate vanished right before my eyes. We...we were camping off shore of that blasted watery hole in the ground and heard some rustlin' nearby. I was sloshed, but I swear to the divines I'm not lying. We went to look at the sound and saw a lass kneeling before the water. Before we could as if she was alright, her body shook with the creepiest giggles ya ever heard. Not even ten seconds later she whipped her head at us and screeched. I just about fell over as my mate stumbled forward. Next thing I know, both were gone!" - A tattered, yellowed journal entry with faded ink and unknown origin. 

Common legend has it that, if you are visiting the void and see an unknown young woman, one of two things can happen. If she is weeping by the water, you should be fine. Take it slow and give her space. However, if upon meeting she begins to laugh, you're out of luck. Many agree you have about nine seconds before she spirits you away, drowns you, or heavens knows what else. Some believe this to be a test of soul. If this ethereal woman senses anything unethical or bad about someone, they have little chance of leaving the void. Otherwise, if there's nothing to hide, you shouldn't have anything to worry about. 
Some popular theories and myths about what lies inside the hole vary from a portal to another realm to a sleeping tentacle monster waiting for its next meal.  Certain religious folk tend to believe it is a realm where their deity dwells. Superstitious or not, even professional divers refuse to explore the dark unknown. Only time will tell if technology advances enough for the void to be scoped out in full.

Type

Underground / Subterranean