Mendenhall Glacier is a glacier about 21.9 km long located in Mendenhall Valley, about 19 km from downtown Juneau in the southeast area of the U.S. state of Alaska. The glacier and surrounding landscape are protected as part of the 2,353-ha Mendenhall Glacier Recreation Area, a federally designated unit of the Tongass National Forest.
Mendenhall Glacier is the most famous of Alaska’s ice floes, attracting hundreds of thousands of visitors each year to see its vast, frozen expanse.
Few visitors, however, see the glacier from its most spectacular vantage point: inside it. Deep within the glacier, there’s a magical blue world of ice caves, unlike anything you’ve seen anywhere else in the world.
The formation of an ice cave starts when water flows through a glacier and melts out a passageway in the ice. The warmer temperature of the water creates a cavity that expands into a cave formation. The opening to this passageway is called a moulin. These small to large holes in the surface of the glacier can run long distances through the ice, sometimes all the way to the glacier’s terminus (face, or end of the glacier) which can sometimes be the ocean itself depending on the location of the glacier. If you have the chance to see a moulin on the glacier, watch your step! These glacier features can be hundreds of feet deep.
However, this interesting phenomenon will fade away if temperatures continue to climb since the head of the glacier will no longer have cold enough ambient temperatures to cause snow to precipitate.
When you step inside, you will immediately notice the glacial ice surrounding you is cerulean blue. The ice appears this color because it absorbs all colors of the visible light spectrum except blue, which it reflects. Thousands of years of packed snowfall create an enormous amount of weight and compress air bubbles out to form ice. The density of glacial ice and transmission of blue wavelengths is what allows this vibrant color and makes it different from other ice formations closer to the surface.
Since the ice melts and shapes differently and glaciers constantly advance or recede, ice caves are never around for long. For that reason, only a lucky few will have the opportunity to explore an ice cave before they disappear and each experience is unique.
According to Wikipedia