The Grand Canyon Caverns are supposedly the largest dry caverns in the United States, located 200 to 300 feet below the surface. You enter the caverns by means of an elevator, dropping 21 stories underground. Down there, the temperature is always at 56 degrees (F), and there is zero humidity.
The cave itself cannot keep up with many of the more spectacular variety, but is nicely done and well worth a visit if expectations are not set too high.
gccaverns.com/tours-activities/caverns/