Mammoth Cave.
The only National Park in Kentucky, it's a vast cave network with around 360 miles of charted caves (and known to have more that they haven't explored yet, including some other cave networks in the area believed to tie in to the caves somewhere).
Basically, if there was a real-world Underdark (or even just a mega-dungeon), this would be it.
Are you aware of any effort to convert Mammoth Cave (or parts of it, at least) into a playable dungeon? I've been looking (which is how I found this thread), and I haven't found anything. I may have to do it myself.
The problem with Mammoth is it doesn't really exhibit a lot of diversity of terrain. You've got a massive fracture line cave right underneath a sandstone cap, so its pretty much a big undecorated maze - and IMO mazes have no real place in an RPG. Other than a map, it's not going to offer much in the way of inspiration.
The other problem you are going to run into is the grottos protect the actual maps of most caves pretty fiercely, just to keep amateurs from getting overly ambitious. Most of the people who have the maps will be part of the caving community and generally unwilling to share them. I don't know if it makes a difference if the cave is a national park, as it might be available under some sort of freedom of information act. But a brief google search did not turn up any real survey maps you could easily turn into dungeons.
Well, yes and no. Compared to other cavern complexes, maybe, most of Mammoth Cave might seem somewhat mundane (being largely, what they call, if memory serves, "living" caverns. But Mammoth encompasses so much terrain that it can't help but be somewhat diverse. There are some truly beautiful "dying" caverns as well (filled with limestone structures. There are impressive heights and depths, caverns large enough to comfortably house an ancient dragon and tunnels tight enough to force kobolds to go squeeze through (although, not on any of the standard tours, naturally).
It's big, I'll give you that. It's just.... well, I have more than an average amount of experience underground, and Mammoth is not where you take anyone if you want them to fall in love with caves. Mammoth is boring in real life, and it would make a boring dungeon.
Water is the life of a cave. A cave is living if it is still subject to natural hydrogeological forces, most especially the removal and deposition of calcite. A cave is dead if these forces are not currently active, owing to the cave having become dry - usually as a result of the water table shifting to a lower level of the caverns as a result of both external and subterranean erosion.