This card features an image of the underground lake that is a waterway for tourists, as indicated by the long flat boats with seating shown at the bottom of the image.
The description states:
CRYSTAL LAKE IN MAMMOTH CAVE
Mammoth Cave National Park, Kentucky
The emerald green water of Crystal Lake provide a colorful contrast to golden tinted walls of the dome above the lake. Mammoth Cave is famous for its underground rivers and lakes.
— This card is one of the current series that dates from the 1950s.