On the western side of Mount Hood lies the longest glacier cave system in the contiguous United States. In 2012, these caves were mapped to a combined length of 7,166.8 feet by cave explorers Brent McGregor and Eddy Cartaya. Currently, the total passage length is hundreds of feet less. Glaciers are frozen rivers; they are always moving and changing... (Photo and caption by Josh Hydeman)

Polish photographer David Kaszlikowski took a series of stunning photographs of a glacier in the Karakoram region of Pakistan while on a film expedition. At the Baltoro and Godwin-Austen glaciers near K2, the second highest mountain in the world, he sent his drone on flights to scout the surrounding landscape. Here: at the heart of Karakoram, a glacier formation found at Concordia at the very beginning of one of the longest glaciers on the planet, Baltoro. (Photo by David Kaszlikowski/Rex Features)

The Aletsch Glacier is pictured at dusk in Fiesch, Switzerland, August 11, 2015. One of Europe's biggest glaciers, the Great Aletsch coils 23 km (14 miles) through the Swiss Alps – and yet this mighty river of ice could almost vanish in the lifetimes of people born today because of climate change. The glacier, 900 metres (2,950 feet) thick at one point, has retreated about 3 km (1.9 miles) since 1870 and that pace is quickening. (Photo by Denis Balibouse/Reuters)

Glacier National Park is a national park located in the U.S. state of Montana, on the Canada–United States border with the Canadian provinces of Alberta and British Columbia. The park encompasses over 1,000,000 acres (4,000 km2) and includes parts of two mountain ranges (sub-ranges of the Rocky Mountains), over 130 named lakes, more than 1,000 different species of plants and hundreds of species of animals. This vast pristine ecosystem is the centerpiece of what has been referred to as the "Crown of the Continent Ecosystem", a region of protected land encompassing 16,000 square miles (41,000 km2).

Vatnajökull (meaning Glacier of Rivers), also known as the Vatna Glacier, is the largest and most voluminous Icelandic glacier, and one of the largest in area in Europe. It is located in the south-east of the island, covering more than 8 percent of the country.

These breathtaking images capture the hidden depths of one of the worlds largest caves, which is so big its home to a beach, a river and a jungle. At more than 130m high, and 150m across, the imposing cave is so big as high as the London Eye and wider than one-and-a-half football pitches. Here: The campers set up inside the cave on the beach. (Photo by Lars Krux/Caters News)

These otherworldly images give a rare glimpse inside caves barely ever seen by the human eye. With its peculiar pools of water and strange colored sediments, the bizarre looking tunnels of the Shakuranskaya cave could be a set straight out of a science fiction film. Found in the disputed region of Abkhazia, around 75 miles outside of Sochi, Russia, it is rumored the incredible underground chambers were formed after a huge earthquake struck the area in 1892. After causing part of a nearby mountain to collapse into the Amtkel River, it is thought the dam-like affect has created a series of interconnecting underground tunnels. Here: a large pool of water inside one of the tunnels. (Photo by Vladimir Mulde/Caters News)

Ra Paulette is an American cave sculptor based in New Mexico who digs into hillsides to sculpt elaborate artistic spaces inside mountains. Reviewer Martha Mendoza in the Los Angeles Times described the caves he created as shrines, as hallowed places, a “sanctuary for prayer and meditation” while others describe the caves as works of art. The caves are finished with “scallops, molded curves, smooth ledges, inlaid stones, narrow pods and crusty ledges”. His caves attract visitors worldwide.