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)

Starting in September, Indonesia's Mount Sinabung began erupting yet again. In the days that followed, ash and smoke were shot into the air multiple times. As a result, nearby towns turned gray from repeated blows by the volcano. Photo: A man uses his mobile phone as Mount Sinabung continues to erupt, with hot smoke spewing from the volcano, in the Karo district on the north of Indonesia's Sumatra island on November 18, 2013. Officials said a week ago at more than 5,000 people had fled their homes since the volcano erupted early this month. (Photo by Sutanta Aditya/AFP Photo)

A robot is seen on Mount Etna, Italy on July 4, 2017. Mount Etna, in Sicily, is a test bed for the approximately three-foot high, four-wheeled machine ahead of a future mission to the moon. It is being conducted by the German Aerospace Centre, the agency which runs Germany's space programme. (Photo by Antonio Parrinello/Reuters)

Construction cranes stand on the Unstrut Valley high-speed railway bridge on March 19, 2012 near Karsdorf, Germany. The Unstrut bridge, which is 2,668m long (about 1.6 miles), is among 38 bridges and 26 tunnels German state rail carrier Deutsche Bahn is building along a new ICE high-speed railroad connection between Nuremberg, Erfurt, Halle, Leipzig and Berlin in a multi-billion Euro project across eastern Germany called the German Unity Transportation Project (Verkehrsprojekt der Deutschen Einheit). (Photo by Sean Gallup/Getty Images)

Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain, near the city of Bihać, Bosnia, was the largest underground airport and military air base in the former Yugoslavia, and one of the largest in Europe. (Photo by Thomas Windisch/Exclusivepix Media)

A Hindu worshipper throws vegetables into the crater as offerings during the Kasada Festival at Mount Bromo in Probolinggo, Indonesia's East Java province, August 1, 2015. Every year people gather for the annual festival where offerings of rice, fruit, vegetables, livestock or money are made to Hindu gods at the active volcano to ask for blessings and assure a bountiful harvest. (Photo by Reuters/Beawiharta)

A round circle of smoke coming from the mouth of the Etna volcano is seen in the sky during an eruption on the southern Italian island of Sicily April 11, 2013. Mount Etna is Europe's tallest and most active volcano. (Photo by Antonio Parrinello/Reuters)

Kelimutu is a volcano, close to the small town of Moni about 50 km to the east of Ende, Indonesia in central Flores Island of Indonesia.