Icicles formed on the roof of caves at Olkhon Island as the waves enter the cave and freeze at the beginning of winter, Lake Baikal, Irkutsk Oblast, Siberia, Russia, Eurasia

Tourist framed in the frozen mouth of ice cave, with icicles formed on roof of caves at Olkhon Island as the waves freeze in winter, Lake Baikal, Irkutsk Oblast, Siberia, Russia, Eurasia

Tourist framed in the frozen mouth of ice cave, with icicles formed on roof of caves at Olkhon Island as the waves freeze in winter, Lake Baikal, Irkutsk Oblast, Siberia, Russia, Eurasia

Bats on roof of cave chamber inside Purah Goa Lawah, Hindu Bat Temple cave, eastern Bali, Indonesia, Southeast Asia, Asia

Pindaya Cave Festival, Pindaya, Shan State, Myanmar (Burma), Asia

Bats on roof of cave chamber inside Purah Goa Lawah, Hindu Bat Temple cave, eastern Bali, Indonesia, Southeast Asia, Asia

View across rooftops to cave houses in the troglodyte district, Guadix, Granada, Andalucia (Andalusia), Spain, Europe

Underground room at Eski Gumusler Monastery, UNESCO World Heritage Site, Cappadocia, Anatolia, Turkey, Asia Minor, Eurasia

Dambulla Rock Cave Temple, UNESCO World Heritage Site, Central Province, Sri Lanka, Asia

Roof of Henjo Cave covered in hundreds golden glowing lanterns. Daishoin Temple, Hiroshima, Japan.

One thousand Buddhas, painted between roof ribs on ceiling dating from 2nd century, Cave 10, Ajanta Caves, UNESCO World Heritage Site, Maharashtra state, India, Asia

Inside ice cave under the Vatnajokull Glacier, with cave guide looking up to hole in cave roof, near Jokulsarlon, South Iceland, Polar Regions

Some of the animals in the Palaeolithic artwork on the roof of the Zoo Cave, Bhimbetka Caves, a large group of rock shelters in quartzitic sandstone in the hills above Obaidullaganj, south of Bhopal, Madhya Pradesh state, India, Asia

Melisani Lake in cave where roof collapsed in an earthquake, Kefalonia (Cephalonia), Ionian Islands, Greece, Europe

Interior of the Gruta do Janelao, a limestone cave passage 100m high, lit through roof window, at Peruacu in Minas Gerais state, Brazil, South America

Melisani Lake in cave where roof collapsed in an earthquake, Kefalonia (Cephalonia), Ionian Islands, Greece, Europe

A caver in Valley Entrance, Kingsdale, Yorkshire Dales, Yorkshire, England, United Kingdom, Europe

A caver in a tight squeeze in the Yorkshire Dales, England, United Kingdom, Europe

12/04/2009. United Kingdom, City of Gibraltar, St Michalel's Cave, Upper Rock Nature Reserve. Stalactites and lime scale formations on cave roof. . City Of Gibraltar, St Michalel's Cave, Gibraltar Strait . UK, United Kingdom

Horse Shoe Bat, cling to cave roof. Mulu National Park, Sarawak, Borneo, Malaysia, Asia

12/04/2009. United Kingdom, City of Gibraltar, St Michalel's Cave, Upper Rock Nature Reserve. Stalactites and lime scale formations on cave roof. . City Of Gibraltar, St Michalel's Cave, Gibraltar Strait . UK, United Kingdom

12/04/2009. United Kingdom, City of Gibraltar, St Michalel's Cave, Upper Rock Nature Reserve. Stalactites and lime scale formations on cave roof. . City Of Gibraltar, St Michalel's Cave, Gibraltar Strait . UK, United Kingdom

Wat Tham Khao Noi, Khao Noi Cave Temple, near Kanchanaburi, Thailand

A pit house, part of an excavated ancient Anasazi village at the Anasazi State Park Museum, the site dates to the 12th century, Boulder, Utah, USA

Stairs to the Pindaya Caves, Pindaya, Burma, Myanmar, Southeast Asia

Unidentifiable climber silhoutted bouldering in Logan Canyon. Utah, USA.

David Gareja Monastery Complex (The 6th Century Rock-Hewn Georgian Orthodox Monastery With Hundreds Of Cave Cells, Churches, Chapels, Refectories And Living Quarters), Kakheti, Georgia

The House On fire ruins at Cedar Mesa, is a natural landmark, a cliff mesa rock formation with a spectacular natural pattern on the rock, Fallen Roof ruins, Cedar Mesa, Utah, USA

The House On fire ruins at Cedar Mesa, is a natural landmark, a cliff mesa rock formation with a spectacular natural pattern on the rock, Fallen Roof ruins, Cedar Mesa, Utah, USA