Here's an image taken last February inside an ice cave in southeast Iceland that extended 300 meters back under a glacier. This an HDR blend of 3 exposures showing the entrance, or rather, the larger "window" above and adjacent to the entrance. To get a sense of the scale, just to the right of the tip of the icicle column is a zodiac boat and a small wooden bridge. The melt stream, very low and frozen solid the day I was there, is visible at the bottom of the image.
I'll be heading back to Iceland to lead my Winter Landscapes, Ice Caves & Auroras workshop March 8 – 14, 2015. There are still 2 spots open, but sign up soon if you're interested in joining me on what is sure to be an incredible photographic adventure.
Below is another shot from the ice cave, looking up through one of the several "skylights" in the ceiling. The opening was about 10 feet above my head and the length of the shaft was maybe another 12 to to 14 feet. Deeper into the cave these shafts would open up 20 to 30 feet overhead and then extend up another 40 to 50 feet up to the top of the glacier. It was one of the most amazing places I've ever visited and I'm looking forward to going back, either to this cave, if it is still accessible, or to another ice cave.