Karstic caves are the commonest type. Slightly acidic water (through natural carbonation) dissolves limestone, marble, dolomite or gypsum. Streams then enlarge the space, carving out long tunnels. Water drips from the roof, evaporating to leave a limestone deposit, which over thousands of years builds up into a stalactite. Similar deposits grow from the ground below as stalagmites, and eventually, the two may unite into a column. This creates "decorated" caves, filled with strange natural architecture and sculpture. Above ground, the outcrops and cliffs are also curiously eroded, so karst countryside is often very scenic. Here and there it's blighted by industry, as the stone is quarried for building masonry, quicklime, cement, chemicals and scores of other uses. Karst country is found worldwide and is riddled with caves, many of which are still being explored or remain to be discovered.

Sea caves are carved out by wave action into fault lines, perhaps supplemented by frost. They're usually fairly short, as the power of the waves is spent within a few metres, but the larger caves are embellished by legends of pirate stash, fugitive princes and sleeping monsters. Striking examples are Fingal's Cave in Staffa, Scotland, and the "Hole in the hat" through Torghatten summit in Nordland - this is 110 m above sea level as post-glacial rebound has uplifted the region. Smoo in Sutherland, Scotland is an unusual hybrid, where a sea cave has become connected to a karstic cave.

Lava tubes result from vulcanism: the outer portion of a lava flow cools, hardens and forms a sheath for the molten interior. That enables the lava to flow further, before draining away when the eruption ceases to leave a tunnel. But this depends upon a particular fluid form of lava. Volcanoes with such lava form molten lakes (caldera) at their summit, and tend to be active continuously but quietly over years, rather than having intermittent noisy eruptions. Hawaii has the best examples of these volcanoes and lava tubes.

Underground works and mines aren't covered in detail on this page, but sometimes a natural space has been extended, and if they have a natural surface they may feel more like a cave than the entrance to the Metro. Notable examples are the "Fred Flintstone" houses of Cappadocia in Turkey, the multi-coloured chambers of Petra in Jordan, the salt caverns of Wieliczka in Poland, the cellar labyrinth of Budapest, and the ice-sculpture galleries of Jungfraujoch in Switzerland.

• 35.75986-5.939331 Caves of Hercules (14 km west of Tangier). Partially natural, partially man made, has functioned as a brothel and a concert venue for the metal band Def Leppard and today probably the most popular place for a quick sidetrip outside Tangier. According to a legend, the Roman god Hercules rested here and supposedly there is an underwater connection across the Strait to Gibraltar. Dh 5. (updated Dec 2015)

• -47.1553-70.65532 Cueva de las Manos ("Cave of Hands") in Santa Cruz province has remarkable prehistoric stencils on its walls.

• -21.144-56.5913 Gruta do Lago Azul Protected natural area in Bonito (Mato Grosso do Sul) featuring a large cave with many unique formations and a large underground lake that takes on a surreal indigo color for much of the year.

• 1.3646-76.22474 Cueva de los Guácharos - National Park in the Andes mountains of Western Colombia, surrounded by rainforest and old oak forests. Karst cave system formed by river systems of the Rio Magdalena and Rio Caquetá.

• -2.995-78.2115 Cueva de los Tayos - Large cave complex in the Amazon rainforest with a number of myths and folk tales surrounding it (including tales of vast mounds of gold).

The Cenotes of the Yucatán cover a wide area of that peninsula. The limestone bedrock is riddled with caves filled with fresh or sea water, so any small pool in a forest clearing may be the entrance to a vast underground network. Cenotes played an important role in Maya religious ceremonies and may have promoted the advance of American civilization - one famous example is called "Car Wash".

• 25.849709-100.5235516 Garcia Caverns (Grutas de Garcia) Series of caverns inside the Sierra de Fraile mountain in northwest Monterrey. Cable car takes you to the top, the walking tour is about 2 miles long and includes several hundred steps.
• 18.66994-99.5088287 Parque Nacional Grutas de Cacahuamilpa Large cavern system operated as a national park in the state of Guerrero. Another cave, Grutas de Pacheco, is located within a few hundred meters of Grutas de Cacahuamilpa.
• 20.671884-98.9224578 Grutas de Tolontongo Commercial caves with waterfalls and warm thermal pools in the Central Mexico town of Ixmiquilpan. Another cave, Grutas Xoxofi is located nearby. 3 hours from Queretaro, 4 hours from Mexico City.
• 18.121727-96.8003819 Sistema Huautla The deepest cave system in the Americas is not a commercial cave, but rather a remote wild cave for the serious cave explorer. In the town of Huautla, Oaxaca

• 37.183-86.110 Mammoth Cave in Kentucky is the world's most extensive known cave system.
• 32.175-104.44411 Carlsbad Caverns in New Mexico no longer hold the record for the world's largest cave chamber, but they're still pretty enormous.
• 40.44-111.70912 Timpanogos are three caves in the Wasatch Range of Utah, linked by man-made tunnels.
• 34.977-85.81413 Russell Cave in northeast Alabama is a remarkable archaeological site, with remains from several cultures going back over 10,000 years.
• 41.714-121.50814 Lava Beds National Monument in the Shasta Cascades region of California has over 800 lava tubes, many of which can be explored, and some of which extend for more than a mile.
• 42.095-123.40615 Oregon Caves of Southern Oregon are formed in marble. That's slower to dissolve, but the chemistry and formation of dripstone decorations are the same.
• 43.729-103.82916 Jewel Cave in the Black Hills of South Dakota has over 200 miles of passages, with sparkling jewel-like calcite deposits.
• 29.8779-97.956617 Wonder World Cave in Texas is a rare tectonic cave, formed by shifting plates along the Balcones Fault; there are no stagmites or stalactites

China's Danxia landscape bears some resemblance to karst; the most famous example is at 25.037113.74718 Danxiashan in Guangdong.

• 25.283110.28719 Guilin and 24.7803110.491420 Yangshuo – The region around these towns in Guangxi (Southwest China) is one of China's most famous and popular tourist areas, primarily because of sensational scenery. The terrain is karst limestone and includes many caves. See the city articles for details on caves that are set up for tourist visits. The area is a major destination for rock climbers, and some of the climbing shops in Yangshuo organise trips to other caves; these all need at least basic rock-climbing skills, and some require advanced spelunking skills and equipment.
• 40.04294.80821 Mogao Caves. A group of caves near Dunhuang in Gansu Province, in the west of China along the old Silk Road. They have many Buddhist frescoes, mostly from a few centuries CE, and are on the UNESCO World Heritage List. (updated Feb 2020)
• 26.585101.71622 Panzhihua. This is a mining town in the south of Sichuan, on the highway and rail line into Yunnan. It is a popular tourist destination, mainly for Chinese travellers. Attractions nearby include forest and mountain areas, and many caves. (updated Feb 2020)
• 24.8191103.325523 Shilin. The "Stone Forest" outside Kunming, the most famous karst landscape in China. 
• 29.117110.48224 Zhangjiajie. Another popular destination for Chinese tourists, famous for karst landscapes, in Hunan province. 

The following areas are considered the caving regions of India: Andhra Pradesh, Chhattisgarh, Haryana, Madhya Pradesh, Meghalaya and Uttarakhand. There are literally thousands of caves in India. There are caves that are man-made (rock cut), others which are religious sites (with or without temples), archaeological sites representing early human life and finally natural caves to explore. Those marked with the symbol are listed in the UNESCO World Heritage List.

• 20.55237775.70043625 Ajanta Caves (Ajantha Caves) (59 km (37 mi) from Jalgaon). 9:00 am to 5:30 pm - Tuesday to Saturday. Ajanta Caves depict the stories of Buddhism spanning from the period from 200 B.C. and 650 A.D. These caves were discovered in the 19th century by some British Officers who had been on a tiger hunt.These 29 caves were built by Buddhist monks using simple tools like hammer & chisel. These caves were the retreats of Buddhist monks who taught and performed rituals in the Chaityas and Viharas, the ancient seats of learning. The elaborate and exquisite sculptures and paintings depict stories from Jataka tales. The caves house images of nymphs and princesses the treasures they contain are a landmark in the overall development of Buddhism (updated Jun 2017)
• 34.214975.500826 Amarnath Cave (Amarnath Temple), Jammu and Kashmir (near Pahalgam). Ice Lingam of Lord Shiva - a stalagmite formation created from water drops from the roof of the cave that freeze. This is considered a holy cave. Except for a period of time during the summer it is covered with snow. Because of the mountainous terrain it is difficult to reach. In addition, there are many rock cut caves (temples) in India that may be of interest. 

• 15.1027578.1114727 Belum Caves, Bellum, Andhra Pradesh. Formed in black limestone, Bellum Caves is one of the largest and longest caves in India. There are stalactite and stalagmite formations, long passages, several large chambers and fresh water galleries. 
• 18.1666678328 Borra Guhalu (Borra Caves), Araku Valley. The Borra Caves are called Borra Guhalu, located in the Ananthagiri hills of the Araku valley on the East coast of India are the deepest cave(s) in India and home to bats, golden gecko, fulvous fruit bat and Insects. 
• 18.90181.9111829 Dandak Cave, Chhattisgarh. Limestone cave located in Kanger Ghati National Park 
• 18.95833372.93055630 Elephanta Caves, Elephanta Island. The Elephanta Caves (Gharapurichi Leni) at Elephanta Island are two groups of sculpted caves. Five Hindu caves make up the 1st group and two Buddhist caves the second group. Winthin the caves there are numerous carved panels and shrines. The original stone elephant stone-cut statue for which Elephanta Island was named is now located in the Jijamata Udyaan (zoo and garden) in Mumbai.  
• 20.02638975.17916731 Ellora Caves. These are an impressive complex of rock shrines that represent the three faiths of Buddhism, Hinduism and Jainism and were created between the 5th and the 13th centuries CE. The caves are 30 km northwest of central Aurangabad, a few km from Khuldabad, and 330 km northeast of Mumbai in Maharashtra state.  
• 25.3335792.6629832 Krem Liat Prah Cave, Meghalaya. Located in the Shnongrim Ridge, Jaintia Hills of Meghalaya, it is the longest cave (25km) yet discovered in India and noted for a large passage called the Airplane Hanger. Meghalaya has over 1000 caves to explore. 
• 18.9066981.9050533 Kutumsar Caves (Cotumsar Caves), Chhattisgarh. Limestone caves located in Kanger Ghati National Park - see also: Dandak Cave 
• 13.0163774.9090134 Nellitheertha Cave (Nellitheertha Cave Temple), Nellitheertha, Karnataka. Located within the Nellitheertha Cave Temple, it is a cave with a low ceiling. Also located here is a small lake and Shiva Lingam. 
• 22.93722277.612535 Rock Shelters of Bhimbetka. They are an archaeological site representing early traces of human life on the Indian subcontinent. There are rock paintings that are similar to those found in Australia and France.  

• -8.0288110.638536 Jomblang Cave (Goa Jomblang), Wonosari, Yogyakarta. A 50-metre vertical collapse-doline cave and a forest at the bottom of the sinkhole. Connected to the Grubug cave, where you can see rays of sunlight shining through the roof of the cave. 

• 10.1663118.91437 Puerto Princesa Subterranean River National Park (Palawan Province). A UNESCO World Heritage Site, 8km (5 miles) of navigable subterranean river with tours by boat available. Usually reached from provincial capital Puerto Princesa, an hour and a half away by bus. 
• 9.654122.99238 Caves near Mabinay (on bus route between two provincial capitals, Bacolod and Dumaguete). This small town is in a mountainous area of Negros Island and has many caves. (updated Feb 2020)

There are also many caves on Samar Island, some accessible to the general public.

• 38.65833334.85361139 Cappadocia. The early Christians dug numerous underground cities — complete with sleeping chambers, food storage, kitchens, wineries, and even an inn for traders — into the soft volcanic soils of the Cappadocia region, to escape raids and persecution. (updated Dec 2015)
• 36.543631.9890440 Damlataş Cave, Alanya. Gorgeous stalactites and stalagmites cover all surfaces of this cave, although some might prefer that it weren't so over-visited. (updated Dec 2015)
• 41.8658227.5866341 Dupnisa Cave, near Demirköy. Deep inside the dense forests of the Istranca Mountains in the northwest of Istanbul, Dupnisa is the home of a bat colony and has a "dry" and a "wet" entrance; a shallow underground stream flows through the latter. (updated Dec 2015)
• 36.4526634.1057942 Heaven and Hell, near Narlıkuyu. Two sinkholes near each other on a hillside looking over the Mediterranean, formed when the ceilings of the respective caves naturally collapsed in. Locals likened the larger of the duo, with its evergreen shrubs on its floor, to "heaven" (and a monastery inside dating back to the 5th century attests to this fact), and the smaller and unexpectedly deep one to an abyss. (updated Dec 2015)

• 40.252758.4396343 Darvaza. In the middle of the remote Karakum Desert of already little-visited Turkmenistan, this man made crater is engulfed by a huge fire burning since 1971, when the Soviet geologists drilling for natural gas in the area accidentally had the ceiling of the cavern collapse. Popularly known as the Door to Hell, it is said to be especially impressive at night. (updated Dec 2015)

• 47.534813.71861 Dachstein Giant Ice Cave
• 47.50277813.1897222 Eisriesenwelt

• 43.72722.58244 Magura Cave in Rabisha, north of Belogradchik, has an extensive gallery of some 750 Neolithic paintings, about 8000 to 10,000 years old.

• 49.36516.72845 Moravský kras in South Moravia has many caves. Five that are publicly accessible are Punkevní jeskyně, Kateřinská jeskyně, Jeskyně Balcarka, Sloupsko-šošůvské jeskyně and Jeskyně Výpustek.

• 62.302321.654446 Wolf Cave (Susiluola) (in Karijoki, Southern Ostrobothnia). The cave has Neanderthal remains from before the Ice Age.  

• 45.04921.175947 Lascaux Caves in the Vézère valley of Dordogne can no longer be visited, to preserve the prehistoric drawings, but they've made an exact replica.

• 49.83411.33248 Franconian Switzerland. Being a Karst region, it has quite a few caves of which some are open to the public on guided tours or for special events like concerts. (updated Feb 2020)
• 49.75472211.4249 Teufelshöhle (Devil's cave), Schüttersmühle 5, 91278 Pottenstein, info@teufelshoehle.de. Summer: 9:00-17:00 Winter: Sunday 11:00–15:00 and on request. A nice limestone cave about 2 km out of the delightful village of Pottenstein with many fascinating rock formations. Adults €4.50 below 15 years: €2.50. (updated Dec 2015)
• 49.81222211.21694450 Binghöhle, 91346 Wiesenttal, info@wiesenttal.de. Summer 10:00–17:00 winter: upon prior request. Adult €4.50 child (4–14): €2.50. (updated Dec 2015)
• 49.826911.375851 Sophienhöhle. Summer: Tu–Su: 10:30–17:00, winter and Mondays (except holidays): closed. Run by the same people that also run Burg Rabenstein, a nearby castle, this cave offers breathtaking limestone formations as well as an honest to god real live cave bear skeleton that was discovered on site. Adult: €5.00 child (4–14): €3.00. (updated Dec 2015)

• 64.748113-23.81794652 Vatnshellir (Snæfellsjökull National Park, Snæfellsnes). The lava tube that features in Jules Verne's novel Journey to the Centre of the Earth.  
• 64.7842-20.72771153 Surtshellir (Borgarnes). Long accessible lava tube, supposedly once home to the fire giant Surtr.  

• 40.560914.205854 Grotta Azzura on Capri is a large sea-cave. Light enters through a narrow, almost-submerged opening, so it's filtered by the sea and suffuses the chamber with a rich blue. Small rowboats can get in, but only in calm conditions at the right state of tides.
• 43.429912.935555 Frasassi Caves near Genga in Marche Region are Italy's finest show caves.

• 67.231415.511456 Svarthamarhola near Bodø is the largest natural cave in Scandinavia, and contains a glacier. It's a strenous hike and you need a guide.
• 67.842212.826357 Kollhellaren on the Lofoten island of Moskenesøya has a cave where prehistoric paintings were discovered in 1989.
• 65.398112.091158 Torghatten is a granite hill on an island near Brønnøysund in Helgeland, with a 160 m passage right through its summit. It was formed by wave and ice action.

• 51.397584.67611159 Denisova Cave (Altai Krai, Siberia). Site where an extinct human species related to Neanderthals were first discovered. Remains are about 40,000 years old and the Denisovans seem to have ranged across much of Asia.  
• 57.441157.004860 Kungur Ice Cave in Kungur, Perm Region, is karst with ice formations within. It's Russia's only show cave.

• 48.57920.54461 Slovak Karst National Park has over 700 caves. The karst spans the border to the Aggtelek cave system of Hungary.

• 45.781214.205762 Postojna Caves near Postojna is one of the largest cave systems in the world, with 20 km of underground galleries, chambers and corridors, up to 50 meters high.
• 46.253113.763863 Migovec Caves in Triglav National Park are almost as extensive.
• 45.662213.988364 Škocjan Caves near Divača holds a huge underground canyon with its own climate.

The UK has a small number of show caves.

• 51.8309-3.686965 Dan-yr-Ogof (Pen y Cae near Swansea). (updated Feb 2020)
• 50.4682-3.50366 Kents Cavern. Home to early man for some 700,000 years, tourists can visit this subterranean wonderland. The caves have attracted many famous people, among them Agatha Christie, Beatrix Potter, King George V of the United Kingdom and Ethiopian emperor Haile Selassie who was so impressed with his visit that he gave his guide, Leslie Powe a gold sovereign.  
• 53.249771-1.92600167 Poole's Cavern (southwest of the town centre). Show cave. A pleasant (and free) walk through woodland from here leads to Solomon's Temple, a folly on a limestone hill with elevated views over Buxton and the surrounding area. Another interesting and little-known local curiosity lies in a small valley between Solomon's Temple and the village of Harpur Hill, where water issuing through lime-rich waste rock has created an intriguing area of white calcite deposits.  
• 51.2278-2.67268 Wookey Hole caves, near Wells. series of limestone caverns open as a show cave/ tourist attraction  
• 56.4314-6.341269 Fingal's Cave. The stand-out sea cave in Britain is Fingal's Cave, a great basalt cathedral on Staffa, near the island of Mull. (updated Feb 2020)
• 58.56311-4.7199770 Smoo Cave (in Durness on the north coast of Scotland). A large sea cave connected to a limestone land cave. (updated Feb 2020)

• -37.036140.79771 Naracoorte Caves near Naracoorte in South Australia have mammal fossils found nowhere else, particularly of Australian Megafauna.
• -33.82150.02672 Jenolan Caves in New South Wales are the most outstanding in Australia. These date back to 340 million years, which make them the world's oldest known cave system.
• -35.7251148.491473 Yarrangobilly Caves are within Kosciuszko National Park in New South Wales.
• -41.6006146.28974 Mole Creek Karst National Park are a set of over 300 caves in Tasmania, although only two are open to the public. One has an extensive network of sediment and bone deposit, while the other is popular for its fantastic glow worm display.
• -34.2386115.04775 Leeuwin-Naturaliste National Park in south-west Western Australia contains over 100 caves, some of which are open for tours. One of which; Jewel Cave contains world's longest straw in a tourist cave with impressively large stalagmites.

If you are permitted take simple photos, take them. You should however be aware that taking photos in low light will require a 'fast' sensor and/or a fast lens. Many shots in a cave will be easiest with a wide-angle lens. Seek appropriate local advice if you wish to use a flash.

Some caves are a habitat for bat species, and with careful planning you may be able to on the surface view the bats leaving a cave (which serves as a roost). Viewing bats leaving their roosts is best done as part of an organized group or with an experienced guide who will be able to advise on how to view without disrupting the normal pattern of the bats, who can be unduly disrupted by lights, noise or certain electronic devices.

Bats are rarely aggressive, but keep your distance since they may bite if threatened. Bats may carry rabies; anyone bitten or scratched by one should see a doctor immediately, and anyone doing a lot of caving should consider getting rabies vaccinations.