A subterranean water formation, Cueva de los Verdes, is seen in Lanzarote, Spain. Rafael Lopez-Monne/NEWSCOM
French vulcanologist Francois LeGuerne (l.), explores and videotapes the steam caves under the glacier ice in the crater of Mount Baker in Washington. Peter Haley/The News Tribune/NEWSCOM/FILE
The inside of a cave is seen here in Fermanagh, Northern Ireland. NEWSCOM
Christina Fritzinger settles in for the night in her snow cave in February 2007 at Snoqualimie Pass, Washington. Fritzinger was a part of 20 students and instructors who received a lesson in snow survival and camping from the Tacoma Mountaineers. The group dug their own snow caves to sleep in for the night. Steve Ringman/Seattle Times/MCT/NEWSCOM/FILE
Stalactites are seen in Carlsbad Caverns National Park in New Mexico. NEWSCOM/FILE
Ice caves in the Perrito Moreno Glacier in Patagonia, Argentina are shown here. NEWSCOM
China's 'Ten-Thousand-Year-Old Ice Cave' is in Ningwu, Shanxi Province. Ice covers the ground, top, and walls of the cave. The site was authorized by UNESCO in 2005. Experts believe the cave formed in the fourth ice age of the Cenozoic era and has more than 3 million years of history. NEWSCOM/FILE
Stalactite formations in a cave are seen here. Stalactites are a type of dripstones and are formed by the deposition of calcium carbonate and other minerals. The stalactites begin with a drop of water, which as it drops down, leaves behind a thin ring of calcite. Each drop leaves another ring so that eventually long, narrow tubes form. These formations can also meet and fuse with other stalactites to create columns over time. NEWSCOM
The subterranean cave systems of Rio Camuy Cave Park in Puerto Rico are among the largest in the Western Hemisphere. Shown here is a sinkhole which suddenly formed, with a drop of 200 feet down to the Camuy River, which flows from the cave entrance. A massive system of tunnels reportedly underlie the area for miles. NEWSCOM
Stalactites and Stalagmites are seen in a cave in Derbyshire, England. NEWSCOM
Rafts are seen outside a cave near Knoxville, Tenn., in June. Jenna Fisher/The Christian Science Monitor/FILE
A man looks at formations in a cave in Cuba in September 2008. The formation is a shield speleothem, which are thick plates of calcite forming boxes, or short stalks with bulbous ends. The dripstone can sometimes project vertically upwards from a cave floor, formed by precipitation from the drips above. NEWSCOM
A woman explores an ice cave on the Franz Josef Glacier in New Zealand. NEWSCOM
An audio recording in English made by Japanese hostage Kenji Goto, seems to extend the deadline for Jordan's release of an al-Qaida-linked prisoner. Jordan had agreed to the prisoner swap with the Islamic State to free a Jordanian pilot being help by the militant group.
The Islamic State group released a message late Wednesday purportedly by Japanese hostage Kenji Goto, extending the deadline for Jordan's release of an Iraqi would-be hotel bomber linked to al-Qaida.