A view of a "cave" dwelling where Chinese President Xi Jinping used to stay, during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Liangjiahe village, Shaanxi province, China May 31, 2021. Picture taken May 31, 2021. REUTERS/Ryan Woo 

People visit a "cave" dwelling where Chinese President Xi Jinping used to stay, during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Liangjiahe village, Shaanxi province, China May 31, 2021. Picture taken May 31, 2021. REUTERS/Ryan Woo 

A wax figure of former Chinese chairman Mao Zedong is seen inside a "cave" dwelling at a tourism site during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Yanan, Shaanxi province, China May 29, 2021. Picture taken May 29, 2021. REUTERS/Ryan Woo 

Villager Guo Jiaming stuffs a stick into a furnace in her cave dwelling in Lin county, Shanxi province, China March 15, 2018. Picture taken March 15, 2018. REUTERS/Joseph Campbell 

A woman sifts through a basket full of dried dates outside her cave dwelling in Lin county, Shanxi province, China March 14, 2018. Picture taken March 14, 2018. REUTERS/Joseph Campbell 

A woman peels a potato from her own field, in her cave dwelling in Lin county, Shanxi province, China March 14, 2018. Picture taken March 14, 2018. REUTERS/Joseph Campbell 

Villager Guo Jiaming, 70, stands outside her cave dwelling on a mountain cliffside in Lin county, Shanxi province, China March 15, 2018. Picture taken March 15, 2018. REUTERS/Joseph Campbell 

The exterior of a villager Li Congdai's cave dwelling is pictured at dusk in Lin county, Shanxi province, China March 14, 2018. Picture taken March 14, 2018. REUTERS/Joseph Campbell 

Villager Guo Lidong harrows a garden in front of his cave dwelling in Lin county, Shanxi province, China March 15, 2018. Picture taken March 15, 2018. REUTERS/Joseph Campbell 

Villager Guo Jiaming, 70, stands outside the entrance to her cave dwelling in Lin county, Shanxi province, China March 15, 2018. Picture taken March 15, 2018. REUTERS/Joseph Campbell 

Villager Guo Jiaming stuffs a stick into a furnace in her cave dwelling in Lin county, Shanxi province, China March 15, 2018. Picture taken March 15, 2018. REUTERS/Joseph Campbell 

Tourists explore cave dwellings of the ancient town of Chufut Kale near Bakhchisaray, Crimea, July 18, 2016. REUTERS/Maxim Zmeyev 

A tourist explores cave dwellings of the ancient town of Chufut Kale near Bakhchisaray, Crimea, July 18, 2016. REUTERS/Maxim Zmeyev 

Tourists explore cave dwellings of the ancient town of Chufut Kale near Bakhchisaray, Crimea, July 18, 2016. REUTERS/Maxim Zmeyev 

Tourists explore cave dwellings of the ancient town of Chufut Kale near Bakhchisaray, Crimea, July 18, 2016. REUTERS/Maxim Zmeyev 

Tourists explore fortress walls and cave dwellings of the ancient town of Chufut Kale near Bakhchisaray, Crimea, July 18, 2016. REUTERS/Maxim Zmeyev 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

Men are seen outside Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

Tourists walk on a street in Matera's Sassi, southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A man stands at a viewpoint from where he can see Matera's Sassi, southern Italy April 29, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 29. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile TPX IMAGES OF THE DAY

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A general view of Matera's Sassi limestone cave dwellings in southern Italy April 30, 2015. With its "Sassi" limestone cave dwellings dug into the hillside and cascading in gravity-defying fashion down a steep slope towards the Gravina river, Matera is one of the Italian cities that time forgot. Picture taken April 30. To match Trip Tips TRAVEL-ITALY/MATERA REUTERS/Tony Gentile 

A replica of pre-historic drawings showing lions is seen on a wall during a press visit at the site of the Cavern of Pont-d'Arc project in Vallon Pont d'Arc April 8, 2015. The Cavern of Pont-d'Arc project is a replica of the pre-historic Decorated Cave of Pont-d'Arc known as Grotte Chauvet, in Vallon Pont-d'Arc in the Ardeche region, containing the world's earliest known art which was recently named a UNESCO World Heritage site. The facsimile cavern, which condenses some 8000m2 of the original site into 3000m2, will display faithful reproductions of paintings and engravings and will be inaugurated on April 10 and will open to the public on April 25. REUTERS/Robert Pratta TPX IMAGES OF THE DAY 

Visitors tour the Fairy Grottoes (Feengrotten) in the eastern German town of Saalfeld, March 19, 2014. Discovered in 1913 inside a former slate mine, the colourful caves are full of stalactites and stalagmites. REUTERS/Fabrizio Bensch (GERMANY - Tags: SOCIETY) 

A Free Syrian Army fighter prepares ammunition inside a cave in Maaret al-Naaman village, in Idlib October 17, 2013. REUTERS/Loubna Mrie (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT MILITARY) 

The interior of a troglodyte cave house is seen in the western mountain town of Gharyan, 100 km (62 miles) south of Tripoli, June 15, 2013. Dug vertically into the ground, the caves are excavated around a central squared courtyard and each room in the dwelling housed a family with cooking areas - smaller caves - used communally. During Libya's 2011 fighting, many families fled to the caves to escape shelling. Troglodyte houses can also be found in other towns in the Nafusa mountains and neighbouring Tunisia. Picture taken June 15, 2013. REUTERS/Ismail Zitouny (LIBYA - Tags: SOCIETY) 

The interior of a troglodyte cave house is seen in the western mountain town of Gharyan, 100 km (62 miles) south of Tripoli, June 15, 2013. Dug vertically into the ground, the caves are excavated around a central squared courtyard and each room in the dwelling housed a family with cooking areas - smaller caves - used communally. During Libya's 2011 fighting, many families fled to the caves to escape shelling. Troglodyte houses can also be found in other towns in the Nafusa mountains and neighbouring Tunisia. Picture taken June 15, 2013. REUTERS/Ismail Zitouny (LIBYA - Tags: SOCIETY) 

A view of a troglodyte cave house in the western mountain town of Gharyan, 100 km (62 miles) south of Tripoli, June 15, 2013. The caves were excavated around a central squared courtyard and each room in the dwelling housed a family with cooking areas - smaller caves - used communally. During Libya's 2011 fighting, many families fled to the caves to escape shelling. Troglodyte houses can also be found in other towns in the Nafusa mountains and neighbouring Tunisia. Picture taken June 15, 2013. REUTERS/Ismail Zitouny (LIBYA - Tags: SOCIETY) 

The Abdullah Monaser has lunch at their home, a cave carved into the rock in a slum of the Akhdam community, near Sanaa March 4, 2013. Abdullah Monaser's family are Yemeni Akhdam, or servants, similar to hereditary castes, and distinguished by their African features and the menial jobs they perform. They reside in carved caves and small huts haphazardly built out of wood and cloth, and make do without basic services such as running water, electricity and sewage networks. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: SOCIETY POVERTY) 

The home of the Hernandez family is seen near San Jose de Las Piedras in Mexico's northern state of Coahuila January 16, 2013. For over 30 years, Benito Hernandez, his wife Santa Martha de la Cruz Villarreal and their family have lived in an odd sun-dried brick home with a huge 40 metre (131 feet) diameter rock used as a roof. The dwelling is found close to the town of San Jose de Piedras, a remote community located in the arid desert of Coahuila, some 80 km (49 miles) from the border with Texas. Picture taken January 16, 2013. REUTERS/Daniel Becerril (MEXICO - Tags: SOCIETY) 

Benito Hernandez holds a DVD of "The Cave", a 2005 horror film from the U.S., while standing inside his home near San Jose de Las Piedras in Mexico's northern state of Coahuila January 16, 2013. For over 30 years, Hernandez, his wife Santa Martha de la Cruz Villarreal and their family have lived in an odd sun-dried brick home with a huge 40 metre (131 feet) diameter rock used as a roof. The dwelling is found close to the town of San Jose de Piedras, a remote community located in the arid desert of Coahuila, some 80 km (49 miles) from the border with Texas. Picture taken January 16, 2013. REUTERS/Daniel Becerril (MEXICO - Tags: SOCIETY) 

Lucero Hernandez, granddaughter of Benito Hernandez and his wife Santa Martha de la Cruz Villarreal, stands in the doorway of her family's home near San Jose de Las Piedras in Mexico's northern state of Coahuila January 17, 2013. For over 30 years, the Hernandez family have lived in an odd sun-dried brick home with a huge 40 metre (131 feet) diameter rock used as a roof. The dwelling is found close to the town of San Jose de Piedras, a remote community located in the arid desert of Coahuila, some 80 km (49 miles) from the border with Texas. Picture taken January 17, 2013. REUTERS/Daniel Becerril (MEXICO - Tags: SOCIETY) 

Former steel factory worker Zarko Hrgic, 57, sits in his home, a cave near Babina river in Babino village, near the central Bosnian town of Zenica, November 3, 2012. Hrgic has been living there since he was deported five years ago for staying illegally in Germany. His day starts with searching for food discarded in garbage bins in cities, as well as cigar butts, from which he makes improvised cigars. Picture taken November 3, 2012. REUTERS/Dado Ruvic (BOSNIA AND HERZEGOVINA - Tags: SOCIETY POVERTY) 

A man is seen in a cave that is used as a shelter, under his house, in Kafruma, an area in the province of Idlib October 26, 2012. Picture taken October 26, 2012. REUTERS/Giath Taha (SYRIA - Tags: POLITICS CIVIL UNREST) 

Residents rest in a stone cave, which becomes a temporary shelters after two earthquakes hit Yiliang, Yunnan province September 8, 2012. Rescuers in southwestern China tried on Saturday to reach remote communities rocked by back-to-back earthquakes that killed at least 89 people and damaged many thousands of buildings, state media reported said. REUTERS/Wong Campion (CHINA - Tags: DISASTER SOCIETY) 

A miner looks on inside the ancient Roman gold mine Catalina Monulesti in Rosia Montana village, 428 km (266 miles) northwest of Bucharest, October 6, 2011. Catalina Monulesti is currently under restoration as a future part of a mining museum. Rosia Montana town, made up of 16 villages that dot the slopes along the river Rosia, has hundred-year-old churches and houses, cemeteries and ancient Roman mine galleries. It also has gold. But for those who live here, that is more of a bane than anything else. Canada's Gabriel Resources wants to build Europe's largest open cast gold mine in Rosia Montana, a 15-year quest that has put the area at the centre of a national debate between heritage and development. Picture taken October 6, 2011. To match Feature ROMANIA-GOLD/ REUTERS/Radu Sigheti (ROMANIA - Tags: BUSINESS COMMODITIES SOCIETY) 

Icicles hang from the ceiling inside the Eisriesenhoehle (giant ice cave) at Dachstein mountain near the village of Obertraun April 28, 2012. Scientists crews led by Oesterreichisches Weltraum Forum (Austrian space forum) tested a space suit technology, three-dimensional cameras, radar, rover vehicles, communications and sterile testing systems during an 11-nation field test in the icy Alpine caves. Picture taken April 28. REUTERS/Lisi Niesner (AUSTRIA - Tags: SCIENCE TECHNOLOGY ENVIRONMENT) 

People walk inside the Eisriesenhoehle (giant ice cave) at Dachstein mountain near the village of Obertraun April 28, 2012. Scientists crews led by Oesterreichisches Weltraum Forum (Austrian space forum) tested a space suit technology, three-dimensional cameras, radar, rover vehicles, communications and sterile testing systems during an 11-nation field test in the icy Alpine caves. Picture taken April 28. REUTERS/Lisi Niesner (AUSTRIA - Tags: SCIENCE TECHNOLOGY ENVIRONMENT) 

Physicist Daniel Schildhammer wears the Aouda.X spacesuit simulator during a field test of Oesterreichisches Weltraum Forum (Austrian space forum) inside the Eisriesenhoehle (giant ice cave) at Dachstein mountain near the village of Obertraun April 28, 2012. The Aouda.X is a spacesuit simulator for manned missions to Mars, which is being developed under the Mars Analog Research Program PolAres and allows to simulate environmental conditions, a real space suit would be faced on Mars. REUTERS/Lisi Niesner (AUSTRIA - Tags: SCIENCE TECHNOLOGY TPX IMAGES OF THE DAY) 

A follower of the Sufi order of Samania enters a cave considered sacred by the order before visiting the grave of its founder Sheikh Bashir Ahmed Tayeb after Friday prayers near the River Nile at Sheikh Bakri village, 135 km (84 miles) north of Khartoum, March 16, 2012. REUTERS/Stringer (SUDAN - Tags: RELIGION) 

A scuba diver visits La Cueva de los Peces (Cave of the Fish) along the coast of Playa Giron, near the Bay of Pigs, 160 km (100 miles) south-east of Havana, in central Cuba January 24, 2012. The Arab Spring, changes in U.S. policy and economic reforms at home are driving a tourist rush that is giving communist-run Cuba one of its best seasons ever and stretching its ability to accommodate demand. Cuba just completed its best year for tourism with 2.7 million visitors in 2011, and experts say current bookings suggest it will almost certainly beat that number in 2012. Picture taken January 24, 2012. REUTERS/Pablo Sanchez (CUBA - Tags: SOCIETY TRAVEL ENVIRONMENT) 

Tourists look at a rock formation inside a cave popularly known in the Philippines as "Underground River" in Puerto Princesa city in the province of Palawan, southwest of Manila, June 28, 2009. A popular tourist destination, the underground river, a UNESCO (United Nations Educational, Scientific, and Cultural Organization) world heritage site, measures 8.2 km (5 miles) long and currently places second in an online public poll for the world's "New 7 Wonders of Nature" campaign. REUTERS/John Javellana (PHILIPPINES ENVIRONMENT) 

Palestinian cave dwellers play with a ball outside their home in a hillside cave in the West Bank village of Ghwien, south of Hebron December 10, 2007. Picture taken December 10, 2007. REUTERS/Nayef Hashlamoun (WEST BANK) 

A Palestinian cave dweller carries a water container into her home in a hillside cave in the West Bank village of Ghwien, south of Hebron December 10, 2007. Picture taken December 10, 2007. REUTERS/Nayef Hashlamoun (WEST BANK) 

A Palestinian cave dweller studies inside a cave at Ghwien village south of the West Bank city of Hebron December 8, 2007. REUTERS/Nayef Hashlamoun (WEST BANK) 

A Palestinian cave dweller walks at her cave home at Ghwien village south of the West Bank city of Hebron December 8, 2007. REUTERS/Nayef Hashlamoun (WEST BANK) 

The sun sets in the valley of Bamian in central Afghanistan September 22, 2005. In 1999, the Islamic fundamentalist movement that was then in control of most of Afghanistan, declared the statues and grottoes idolatrous and un-Islamic, and in March 2001 they used artillery and explosives to demolish the Great Buddha and another large statue. Picture taken on September 22, 2005. REUTERS/Desmond Boylan 

An Afghan boy plays near dwellings carved into a rock in the valley of Bamian in central Afghanistan September 22, 2005. In 1999, the Islamic fundamentalist movement that was then in control of most of Afghanistan, declared the statues and grottoes idolatrous and un-Islamic, and in March 2001, they used artillery and explosives to demolish the Great Buddha and another large statue. Picture taken on September 22, 2005. REUTERS/Desmond Boylan 

A picture shows the grottoes carved in the rock of the valley of Bamian in central Afghanistan September 21, 2005. In 1999, the Islamic fundamentalist movement that was then in control of most of Afghanistan, declared the statues and grottoes idolatrous and un-Islamic, and in March 2001, they used artillery and explosives to demolish the Great Buddha and another large statue. Picture taken on September 21, 2005. REUTERS/Desmond Boylan