The Pit

Surface caves aren't all that unheard of-- Small little recesses breaking down into the earth have acted as crude forms of shelter for many an adventurer, and more often than not the beast that lurks just a little further in since you forgot to do a full sweep of the entire cave before setting up your camp, ya big dummy.
This one is.. A little more than that. Sitting on the intersection between the dwarven homelands of Waradin, the deliberately ambiguous area known as Something, and the Base of the Beast, there lies one of the largest chasms across the surface of Vertinall (at least one that isn't currently being occupied by a giant Snake.)
The Endless Abyss, The Beast's Doorway; I just call it what it is. The Pit.

The Great Map of Vertinall

Open full size

Just a normal map, nothing to see here.

It's a hole, big deal

That 'Big Deal' is a gigantic hole punched from one end of the world to the other, with the endless vaccum of space and nothingness sitting beneath it. Maybe it's the latent primitive instinct in me to see a big old hole and go 'Woah, that's pretty cool' but I think it's earned it's place as somewhere interesting, especially given it's dubious origin, and talent for eating adventuring parties that attempt to explore it. While you could argue that a big old hole in the ground doesn't warrant much reason for exploring, the real mystery lies in the countless openings that sit within it's inner walls.
There are various tunnels that can be spotted scattered across the inside of The Pit itself, rumoured to be an intricate network of tunnels that connect across the entire planet. These pathways have been a point of study for many, as the frequency of which they appear has been deemed to be 'too convenient' to be naturally occurring, and is an indication of another civilisation altogether.

Where did it come from?

The historical significance of The Pit has yet to be fully established, but there is much speculation. The most favoured theory people have accepted thanks to the Church, is that it was a hole caused by The Beast itself during it's emergence. People with any vague amount of intelligence decided to measure the area around The Pit, and for the Beast to have been the cause, he would have had to have been much, much smaller. Mentioning as much to them usually has them grabbing torches and pitchforks, for daring to even suggest that the Beast was ever at any point "Small" (If you can count an impossibly giant hole small...), so really it's still up for debate.
Then there's the series of tunnels poked throughout the whole thing. Were they dug out? Naturally occurring? There could be a veritable world of knowledge that was lost to Vertinall long ago hiding away beyond the dark, and all it would take is a brave few to set off and explore, and return with their findings.

So why haven't they?

Many-- And I really do mean many, have attempted to explore the Inner caves within the Pit, and not a single person who has entered them has ever returned. The earliest attempts tried to answer the simplest question:

"Why not just fly in?"

— Probably one of the earliest attempters

It was unfortunately very quickly discovered that any means of entry beyond climbing were strictly off the table, when handfuls of winged and magically-inclined inhabitants discovered their ability to fly was quickly snatched away from them. It seems that beyond a certain point in the Pit, all means of flight, both Magical and Natural, cease to function; As if the air carried beneath them simply.. Vanishes.
Contrived as hell, right? It's stuff like that making folks think there's something worthwhile down there, instead of seeing it as the warning to stay away that it actually is. And so, to this day people still make their attempts; Anchoring rope ladders to clamber into the nearest entry point only to have materials holding the ladder together fall away just as they step into the cave, forcing them to move forward.
Now either there's a really great party going on down there that's compelled people to stay in the tunnels for the last few thousand years or so, or there are a lot of corpses littering those halls. Unfortunately, that too encourages people to explore, snatching up the potential loot left behind by other failed attempts, so the Pit is never left too long without it's next batch of Victims.

Another another problem

There's another explanation for all of the disappearing adventurers, and it's unfortunately one that even the more scholarly of minds find difficult to refute.
There is the possibility that these interconnecting tunnels are the hollowed out channels that belonged to the Leylines, a formidable form of natural magic that has all but been lost to Vertinall, save for the dregs of energy left behind, which inhabitants now know to be called Anything. If even a hint of that magic's presence lays within those halls, it would all but certify it's authenticity as an impossible death trap.

A Warning to any prospective Adventurers:
The contents of this article may entice you to attempt your own exploration of The Pit, on some deluded notion that you will be the one to break the Cycle and to succeed where every single other person has failed, so allow me to make this painfully clear.
The only treasure that waits for you in The Pit is Death.
Proceed at your own free-will, but with the knowledge that you were offered the chance to turn away, and refused.

"I don't have enough fingers to count the amount of students that have come poking around and asking about exploring The Pit. They tend to stop when I tell them that I have about ten too many fingers to count the amount who have ever come back from their own attempts."

— Seluavir Seluaval, Gnomish Professor of the Adventuring Academy

by TheDumbOwl (Me)

How many other ways can I spell this out for you? Whatever reward sits at the end of those tunnels just isn't worth it-- no matter how groundbreaking. Because even if you do happen to reach it, there's almost no chance at all that you'll ever make it back out. This one is best left in the books as an unsolvable mystery.

Young one...
Be wary of the Speaker. He hides the truth of this place from you. He believes you are weak.
Defy him. Find us. Free us.