I wasn't prepared for this experience. The NPS guidebook said the main cavern was "The size of six football fields" and it has holes in the floor which go down 150 feet, as well as ceilings which reach 200 feet above the floor. . . . and the whole thing's 700 feet underground. The stalactites are amazing . . going from the tiny - a bout 1/4 inch to the large, but it's the stalagmites, the towers which rise off the floor, which get truly enormous, as you can see. The finest stalactites, called 'soda straws' (because the calcium-laden wated flows down the inside of them) make wonderful patterns and hanging sculptures.
Most of the pictures here were taken under available light, with long exposures, and the ISO rating cranked up to 1600. I tried a few with flash, at 2 stops underexposed, but it just bleached everything out and removed any vestiges of depth and detail.
If you're ever planning to visit the Southwest of the US, don't miss this!