Rainy Chamber is named from the fact that during the early months of summer water falls constantly in the form of a light shower; but it drips at all times, and in consequence there is an opportunity to study the active process of formation of one of the deposits which is very abundant in Wind Cave and considered the most perplexing. This is the pop-corn, and the theories of its origin have been steadily rejected at Wind Cave because of a doubt being entertained as to whether it has been deposited under water or by drippings. Here in Rainy Chamber it is fully explained. Near the center of the room the fallen masses are heavily crystallized, much of the groundwork being fine box work and the crystals in perfect condition. On these crystals the pop-corn is being formed, and specimens can be seen in all stages of development, from the beginning to an approximate degree of finish; and whatever the position it occupies on the receiving surface, either on top, underneath, or on a side exposure, it always maintains the same relative position as growing plants on the mundane sphere. The water falling on the upper surface in scattering drops forms myriads of minute stalagmites; on side positions the falling drop first strikes the point exposed to its line of descent and then spreads. The scant moisture slowly makes its way down sloping sides and shelving edges, leaving on each small irregularity a tiny portion of its volume, to deposit an infinitely small charge of solid substance, and the balance finally hangs in moisture less than drops on the growing grains of the under surface. Pop-corn, therefore, is the globular aragonite of the stalagmitic variety. A small specimen from Rainy Chamber, placed beside one of the same color from Wind Cave, shows them to be absolutely alike.