So what have you seen in the real world that is too cool for the D&D world?
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).
It includes
Vast chambers (the largest is 2 acres in floor size, and goes up about 30 or 40 feet, and there are about 6 chambers known only slightly smaller than that)
An underground river. (Referred to informally as the River Styx)
An abandoned city (a colony for treatment of tuberculosis in the 1800's (they thought the cave air would help, so dozens of people lived in the caves full-time with their doctor in an underground city),
Secret chapels (apparently a few old-time preachers liked to hold services in the cave, preaching from atop a cliff to a big chamber of followers),
Mummies. Mummified remains of deceased Native Americans have been found in there),
Abandoned mines (saltpeter and gypsum mining used to be big in that network 100+ years ago)
Cryptic ruins & relics. There are many sets of artifacts known to be several thousand years old, including clay pots and stone weapons.
Secret rooms. During the slavery era, apparently some slaves met in these chambers, well off the beaten path and not easily seen by lantern light, to be illicitly taught to read & write, by using charcoal and writing on the cave walls.
The "Star Chamber" (A vast chamber where shining specks of crystal peek out from the stone, so by torch or lantern-light, it creates the illusion of the ceiling sparkling like the stars in the night sky.
An underground waterfall. Water falls from some place above, draining from the surface and falling over a hundred feet before it hits the surface of a pool by the side of what is essentially an underground beach (gently sloping shore covered in very fine gravel)
My wife and myself did a lantern-light tour there a few weeks ago. It was walking through a 3 mile stretch of cave by oil lantern with a tour group.
Basically, if there was a real-world Underdark (or even just a mega-dungeon), this would be it.