So that's what's at the end of the rainbow! These are the stunning pictures that show light streaming into a deep cave - while abselillers lower themselves down into the drop.
The amazing photographs appear to show daredevil explorers hanging by a thread as they plunge into the darkness of the caves - bathed in beautiful colourful light.
The shots show cavers lowering themselves into Stephen's gap - a geological wonder in Alabama, USA.
Daredevil photographer Amy Hinkle had to be strapped up in safety gear as she balanced on a slippy ledge inside the cave to capture the images.
Amy, from Chicago, said: "I have been a caver for over two years and enjoyed photography for much longer. Getting photos in the underground world is entirely different, however. There is no light but what you bring with you, except near entrances, of course, where you get some sunlight.
"I have worked hard to develop techniques to bring light to these pitch-black underground systems without the use of massive amounts of equipment.
"My photo gear fits into the size of two hands cupped together, weighs less than three pounds, and cost under $200 (£130). I enjoy challenges like this which combine my scientific and artistic backgrounds.
"I saw a rainbow on my first trip to Stephen's Gap over a year ago, so I knew it occurred, and planned to take some pictures.
"However, I did not expect the rays to be so clear and the rainbows so bright! The cool air from the cave hitting the hot outside air - which was about 35 degrees centigrade - produced that amazing fog layer which distributed the natural light perfectly.
Stephen's Gap is a solution cave, formed by water carving away at the limestone rocks ages ago.
As changes slowly occur over a geological timescale, entrances may appear by means such as surface collapse - when the remaining rock can no longer support the surface, like when sinkholes occur - or sometimes where water once entered - becomes a dry streamed that is humanly passable into the cave.