18. Cliff cities of the southwest.
Instead of the (IMO) silly lost egyptian temple in the grand canyon, try basing a dungeon off of one of the empty cliff-cities of the American Southwest, the ancient dwellings of the Anasazi, the ancestors of the Hopi tribes that live in the area now. Perhaps the mortal inhabitants left a dark reminder of what drove them from the cities in the first place, or perhaps they have returned, and are not what we have long thought them to be. Even easier, just assume that a modern group has moved in and claimed the city as their own, filling the ancient, mysterious structures with modern traps and technology.
19. Masonic Temple/Lodge
Large meeting places for the group most blamed for conspiracies in the world, these buildings tend to look like medieval citadels in urban cityscapes. Laborinthine structures with lots of symbolic architecture, these make for great "dungeons." Watch out for secret pyramid symbolism, all American leaders since WWII, and erotic-satanist rituals: all a part of the modern conspiracy mythology that belong here.
20. Off shore Oil Rig
Huge, isolated structures with cramped, tight hallways and few ways to contact the outside world. Chase a bad guy here, or find something that Man Was Not Meant to Know that the drilling has disturbed in the ocean floor. Whatever floats your boat.
21. Secret Nazi Base beneath Antarctica
(IMO) Unlikely to actually exist, this would nonetheless make an awesome adventure for D20 modern in which we bend reality, just a tad. It could be filled with everything from futuristic UFO technology to inbred mutoid lunatics forced to survive on supplies left here at the end of WWII. Either way, it's bad.
22. Mammoth Cave
Real-life cave system that stretches beneath most of the mid-western United States (from Kentucky to Kansas and Missourri at least), it's an underdark with its own ecosystem. Only a small percentage of the caves and tunnels are mapped, making this the potential home of nearly everything in your Menace Manual, or the gateway to a "Hollow Earth."
23. Secret Volcano Lair
Taking a cue from Bond books and films and Austin Powers, this is COMPLETELY unrealistic, but still makes a good entry for a dungeon crawl of some sort.
24. Ancient Egyptian Necropolis (just being excavated)
See Raiders of the Lost Ark, and the Mummy. I need not say more.
25. Early Christian sanctuaries/Tombs
Indiana Jones and the Last Crusade.
26. Ancient Indian Temples
Temple of Doom
27. Secret Tunnel System Beneath Glastonbury Tor (England)
New Age mystics and UFO enthusiasts alike have reported that there are strange things (including the Holy Grail!) beneath Glastonbury Tor in western Britain. Whether there are aliens or the cup of Christ hidden below the hill, your adventurers can hack and slash their way to discovery--assuming they can find the secret entrance.
That's all I have right now. I'll think of more later.