A cave or cavern is a natural underground space large enough for a human to enter. Caves form naturally by the weathering of rock and they often extend deep underground. Because of their uniqueness and beauty, some caves have become very interesting places for tourists and photographers worldwide.

1. SEA CAVE IN ALGARVE, PORTUGAL

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This beautiful sea cave is located near Benagil Beach in the Algarve region of Portugal. As you can see the bottom of the cave is covered with fine sand, and on the roof there is a “window to the sky”. [map]

2. CRYSTAL CAVE, ICELAND

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This cave in the glacier ice is the result of glacial mill, or Moulin where rain and melt water on the glacier surface are channeled into streams that enter the glacier at crevices. The waterfall melts a hole into the glacier while the ponded water drains towards lower elevations by forming long ice caves with an outlet at the terminus of the glacier.
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The fine grained sediments in the water along with wind blown sediments cause the frozen meltwater stream to appear in a muddy colour while the top of the cave exhibits the deep blue colour. Due to the fast movement of the glacier of about 1 m (3 ft) per day over uneven terrain, this ice cave cracked up at its end into a deep vertical crevice, called cerrac. This causes the indirect daylight to enter the ice cave from both ends resulting in homogeneous lighting of the ice tunnel.
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The cave is accessible through a 22-foot (7 m) entrance on the shoreline. At the end it tapers to a tight squeeze no more than four feet high (1,2 m). Ice caves are in general unstable things and can collapse at any time. They are safe to enter only in winter when the cold temperatures harden the ice. Even so one could hear constant cracking sounds inside the cave. It was not because it was going to collapse but because the cave was moving along with the glacier itself. Each time the glacier moved a millimeter loud sounds could be heard. [link, map]

3. DEVETASHKA CAVE, BULGARIA

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Devetashka cave is one of the biggest caves in Bulgaria but it is most famous for its amazing history. It is also currently home to nearly 30,000 bats.
The man on the floor of the Devetashka cave  link
Devetashka cave is located 18 km (11mi)  northeast of Lovech and 2 kilometers (1.2mi) away from the village of Devetaki. The cave is also known as Maarata or Oknata for its seven different-sized holes in the ceiling, through which sunlight penetrates and illuminates the central hall and part of its two fields.
Old building inside of entrance area  link
The entrance of the cave is 35 meters (115ft) wide and 30 meters (100ft) high. About 40 meters (130ft) after the entrance, the cave widens, forming a spacious hall with an area of 2,400 sqm (0.6 acres). The height of the hall is 60 meters (197ft); at some places it reaches 100 meters (330ft). [link, map]

4. GLOW WORM CAVES, NEW ZEALAND

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Waitomo, New Zealand is famous for one thing. Every year, heaps upon heaps of travelers come here to see the infamous glow worms that line the ceilings of the nearby caves. The glow worms emit a phosphorescent glow that shines from the roofs of the caves like a starry night.
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The interesting thing about the glow worms is that they aren’t really glow worms. They’re fly larvae. And what glows? Well, that’s their waste and snot. The larva glow to attract prey into its threads by making the prey believe they are outdoors as the roof of a cave looks much like a starry night. Hungry larva glow brighter than ones that have just eaten. There are a couple of ways to see the glow worms. There’s the three hour black water tubing trip, the five hour trip that includes abseiling and climbing, or, if you like it easy, a boat. [link, map]

5. CUEVA DE ARPEA, SPAIN

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Cueva de Arpea is located below the twisted rock layers in Aezkoa valley in northern Spain. That area is home to some truly amazing caves.
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The cave is called in french “caverne d’harpea”. It’s believed to have been used as a refuge by ancient shepherds since old times. [map]

6. MINNEHAHA FALLS CAVE, USA

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Near where the Minnehaha Creek meets the Mississippi River, a 53 ft (16m) waterfall that freezes during the winter creates a temporary cave behind a wall of ice. Due to the extremely cold temperatures in the area during the winter months, the falls freeze, creating a dramatic cascade of ice that can last well into the spring.
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Minnehaha Creek is located in Hennepin County, Minnesota that extends from Lake Minnetonka in the west and flows east for 22 miles through several suburbs west of Minneapolis and then through south Minneapolis. Including Lake Minnetonka, the watershed for the creek covers 181 square miles.
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The creek might have been unremarkable except for the 53 foot (16m) Minnehaha Falls located near the creek’s confluence with the Mississippi. The site is not far from Fort Snelling, one of the earliest white settlements in the region. [link, map]

7. FINGAL’S CAVE, UK

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Fingal’s Cave is a sea cave on the uninhabited island of Staffa, in the Inner Hebrides ofScotland, part of a National Nature Reserve owned by the National Trust for Scotland. It is formed entirely from hexagonally jointed basalt columns.
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Its size and naturally arched roof, and the eerie sounds produced by the echoes of waves, give it the atmosphere of a natural cathedral. The cave’s Gaelic name, An Uaimh Bhinn, means “the melodious cave“.
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The cave has a large arched entrance and is filled by the sea. Several local companies include a pass by the cave in sightseeing cruises from April to September. However, it is also possible to land elsewhere on the island and walk to the cave overland, where a row of fractured columns forms a walkway just above high-water level permitting exploration on foot. [link, map]

8. GIANT CRYSTAL CAVE, MEXICO

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Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres (980 ft) below the surface in Naica, Chihuahua, Mexico. The main chamber contains giantselenite crystals (gypsum, CaSO4·2 H2O), some of the largest natural crystals ever found.
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The cave’s largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time.
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A group of scientists known as the Naica Project have been heavily involved in researching these caverns. [link, map]

9. CENOTE IK KIL, MEXICO

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Ik Kil is a well known cave with no roof, or a sinkhole outside Pisté in the Municipality of Tinúm, Yucatán, Mexico, It is located in the northern center of the Yucatán Peninsula and is part of the Ik Kil Archeological Park near Chichen Itza. It is open to the public for swimming and is often included in bus tours.
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This cave is open to the sky with the water level about 26 metres (85 ft) below ground level. There is a carved stairway down to a swimming platform. Ik Kil is about 60 metres (200 ft) in diameter and about 40 metres (130 ft) deep. There are vines which reach from the opening all the way down to the water along with small waterfalls. Also, there are black catfish which swim in the sinkhole. [link, map]

10. BLUE LAKE CAVE, BRAZIL

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Mato Grosso do Sul region in Brazil (and especially the quiet town of Bonito) boasts many marvelous underground lakes: Gruta do Lago Azul, Gruta do Mimoso, Aquário Natural. The world famous “Gruta do Lago Azul” (Blue Lake Cave) is a natural monument whose interior is formed by stalactites, stalagmites and a huge and wonderful blue lake.
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The beauty of the lake is something impressive. The Blue Lake Cave has a big variety of geological formation but impresses mainly for the deep blue colored water of its inside lake. [link]

11. BLUE MARBLE CAVES, CHILE

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In Patagonia, South America, General Carrera Lake is shared by Argentina and Chile. But on the Chilean side of the beautiful emerald-green to turquoise-blue waters, there are breathtakingly beautiful marble caves carved into passageways and caverns.
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These amazing marble formations were sculpted by erosion into three main marble formations: La Capilla (the Chapel), El Catedral (the Cathedral), and La Cueva (the Cave).
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The impressive labyrinth of marble caves are large enough for a small boat to glide into. [link,map]