Location: Hang Son Doong

Another post from the "23 Unbelievable Places" series, with credit to Reddit user Boatakk. I think this will be the last one from that album for a little while, though I do intend to finish it out.


Description:
The cavern is immense, and seems to continue downward forever. The walls stretch at least 100 feet high, and the stone slopes and bends all throughout the cave system. As you descend deeper and deeper, the light from the entrance wanes, and is slowly replaced only by the phosphorescent fungi that live in the depths. The air is damp and cold, and this intensifies as you go deeper. Massive stalagmites interrupt the descent every now and then, and you can just make out the faintest sounds of life as you approach. 

Associated Knowledges:
Nature:

• This cavern, or at least part of it, was carved out by water flowing for centuries until an opening formed on the surface.
• Even by just glancing at the first entrance you see, you can know that this cave system must span miles of the surface, but you can't tell how deep it goes.
• Something about this cave is definitely not natural, and this feeling intensifies as you go deeper.

Dungeoneering:

• The entrance to the cave is sloping steadily downward.
• The entrance to the cave appears to be made of natural stone.
• This cave goes deeper than any you have ever heard of - but you can not tell just how deep that may be.

Planes:

• The entrance to the cave appears to be anchored readily in the material plane.
• You can detect a faint aura of planeshifting that gets stronger the deeper you go.
• The cave appears to be a passageway between the material plane and the Underdark.

DM Info:
  Hang Son Doong is an entrance into the Underdark from the material plane. Depending on what level the PCs are, this could be completely undiscovered from the denizens of the Underdark, or could be used as a major route for invaders. In either case, the cave should be incredibly dangerous the deeper one goes. The cave itself was formed completely naturally, and has been growing steadily for thousands of years. However, as it got deeper, the cave actually formed it's own rift into the Underdark, seamlessly joining the two planes together. The cave itself is massive, and covers some few hundred square miles of territory on the surface. The entrances to the cave are somewhat randomly scattered. They all are interconnected, but not necessarily directly - some tunnels might be a straight drop into the Underdark, others may be a gentle slope, and others still could be some combination of the two. The cavern is so massive that an entire army could feasibly move through it, assuming they could deal with the difficult terrain. This will, of course, play a significant role.
  The cave itself is not that interesting, aside from being very large. It is a cave, with stalagmites, stalactites, random pits, and made of stone. It is natural, and so has no rational structure. It generally always slopes downward, but there could be some spots where it's more of a random scatter, or could be more or less level. The cave itself doesn't necessarily have structured "floors," but you could certainly have the PCs go down some sort of tiered pathway if you so desired. It is effectively a mega-dungeon that never ends. It directly leads into the Underdark, which in and of itself is a massive dungeon of sorts. 
  The deeper the PCs go, the more magically attuned the cave becomes. Fey and Nature magic will become more evil-aligned just due to the "nature" of the Underdark, pun fully intended. Different parts of the cave don't necessarily resonate with any particular type of magic, although the system itself is random enough that there certainly could be one or two areas that coincidentally did so. Perhaps there would even be some sort of crystal formation growing there, as a natural response to the magic, as an indicator to the PCs that it's important. Actually, I like that idea, so there are some areas of the cave that resonate with magic and will have weird crystal formations to indicate that. 
  The wildlife of the cave varies depending on how deep you are. On the surface, the cave has very normal cave-dwelling creatures. Goblins, Kobolds, bears, etc. The deeper you go, however, the more dangerous and magical the creatures can become. Carnivorous plants, fiendish bats, giant spiders. Deeper still you will find Drow who might not know of the cavern's importance. Even more scary would be Drow that DO know of the cavern's importance, and intend to use it for their own benefit. 
I like this idea because the cavern presents a very easy plot device. The players might discover the horrible nature of Hang Son Doong, and attempt to cover it up. Or perhaps they never investigated that weird cave, and all of a sudden the Drow are staging a massive invasion of the surface world. Also, it's a giant cave, which more than one monster or adventurer has probably attempted to hide their loot in, only for it to be lost. 

Adventure Hooks:

• A number of bandit raids are coming from an area that has a lot of caves. The PCs then discover that all the caves are interconnected.
• Rumors have spread that long ago, the caves were actually a nest for the Formians, and they had treasure buried deep below.
• A powerful Drow warlord has "surfaced" - pun again fully intended - and is using the caverns as a base of operations to stage his invasion of the surface world. The PCs are tasked with closing this gateway forever.