Old Postcard: Mammoth Onyx Cave

The back of this postcard note:
The ease with which the visitor steps directly from a cedar-clad hillside, with blue sky and waving tree-tops overhead, into this gloriously tinted underground world, suggests that one has used the magic "Open Sesame" to gain admission.

Horse Cave Kentucky. See the cave or the museum, and catch a play by professional actors. This is a very cool and a very underrated area.