Caves and tunnels have always been part of human life. We've grown more adept at shaping these underground shelters and passages over the millennia, and today we dig for hundreds of reasons. We excavate to find both literal and cultural treasures, digging mines and unearthing archaeological discoveries. We use caverns for stable storage, for entertainment, and for an effective shelter from natural and man-made disasters. And as the planet's surface becomes ever more crowded, and national borders are closed, tunnels provide pathways for our vehicles and for smugglers of every kind. Collected below are more recent subterranean scenes from around the world.
A student from the University of Indonesia descends into Jomblang cave at Gunungkidul district, near the ancient city of Yogyakarta, on June 20, 2012. Jomblang cave is one of the hundreds of caves in the Gunungkidul district. Jomblang is known for its fertile and dense vegetation and is located in the karst hills that run along Central Java to West Java provinces.
The upper cave of the Jeita Grotto north of Beirut, Lebanon, on November 10, 2011. Jeita Grotto is a group of caves located 20 km north of Beirut in the Valley of Nahr al-Kalb (Dog River). In these caves and galleries the action of water in the limestone has created cathedral-like vaults of stalactites and stalagmites, stone curtains and fantastic rock formations.
(AP Photo/Hussein Malla)
A tourist walks through an underground tunnel archaeologists say is a 2,000-year-old drainage tunnel, leading to Jerusalem's Old City, on August 2, 2011. The excavation of the drainage tunnel beneath Jerusalem yielded new artifacts from a war here 2,000 years ago, archaeologists said.
(AP Photo/Dan Balilty)
A man inspectas a sinkhole formed in a house on July 19, 2011 in the north of Guatemala City. When neighbors heard a loud boom overnight they thought a cooking gas canister had detonated. Instead they found a deep sinkhole inside a home. The sinkhole was 12.2 meters (40 feet) deep and 80 centimeters (32 inches) in diameter, an AFP journalist who visited the site reported. Police, members of the country's natural disaster office and water utility company officials came to visit the site. Sinkholes, formed by the natural process of erosion, can be gradual but are often sudden. Guatemala City, built on volcanic deposits, is especially prone to sinkholes, often blamed on a leaky sewer system or on heavy rain.
(Johan Ordonez/AFP/Getty Images)
Aviam Atar, of the Israel Nature and Parks Authority, enters a steel cavern, part of an abandoned Israeli army bunker during a media tour for Reuters near the West Bank city of Jericho, on January 23, 2012. The abandoned Israeli army bunkers along the Jordan River are providing a lifeline for bats on the endangered species list, researchers say.
The pavement of Route 61, eroded and covered in graffiti in Centralia, Pennsylvania, on May 24, 2012. Fifty years ago, a fire at the town dump spread to a network of coal mines underneath hundreds of homes and business in the northeastern Pennsylvania borough of Centralia, eventually forcing the demolition of nearly every building. The fire still burns beneath some 400 acres, and may continue to burn for another 250 years.
(AP Photo/Michael Rubinkam)
Some of the 20 million bats emerge from Bracken Cave in Bracken, Texas, on August 31, 2011. A depleting insect population has forced millions of bats around drought-stricken Texas to emerge before nightfall for food runs, making them more susceptible to natural predators. Some experts have already noticed fewer bats emerging from caves and have seen evidence that more infant bats are showing up dead, hinting at a looming population decline.
(AP Photo/Eric Gay)
A sign welcomes visitors to a lab 4,850 feet beneath the earth on Wednesday, May 30, 2012. The Sanford Underground Research Facility in Lead, South Dakota, will house the world's most sensitive dark-matter detector. Scientists say that the lab -- housed inside the now-shuttered Homestake Gold Mine -- could help scientists understand the origins of the universe.
(AP Photo/Amber Hunt)
An employee of AlpTransit Gotthard Ltd and a visitor stand in the NEAT Gotthard Base tunnel near Erstfeld, on May 7, 2012. Crossing the Alps, the world's longest train tunnel should become operational at the end of 2016. The project consists of two parallel single track tunnels, each measuring 57 km (35 miles) in length.
Construction workers lift a piece of the ceiling off an underground tube that the Saw Mill River flowed through in downtown Yonkers, New York, on November 1, 2011. The Saw Mill River, which was covered over in Yonkers in the 1920s, is in the midst of a "daylighting" process, being uncovered and integrated into a riverside park in the downtown of the city.
(AP Photo/Seth Wenig)
View from inside a tunnel recently found in the northern border city of Tijuana, Mexico, on November 30, 2011. A day earlier, the tunnel was discovered by U.S. authorities in San Diego's Otay Mesa area, the latest in a spate of secret passages found to smuggle drugs from Mexico. This tunnel is a 400-yard passage linking warehouses in San Diego and Tijuana and is equipped with lighting and ventilation.
(AP Photo/Alex Cossio)
A municipality worker looks away from the welding glare as another repairs an underground water pipeline in Mumbai, on October 16, 2011. The water distribution system in Mumbai is over 100 years old. Water is brought in from six lakes after treatment, and stored in 23 service reservoirs.
A staff member of the metropolitan outer floodway management office looks around pressure-adjusting water tank, a part of a massive underground water discharge tunnel which was constructed to protect Tokyo and its suburb area against floodwaters and overflow of the city's major waterways and rivers during heavy rain and typhoon seasons, at the facility in Kasukabe, north of Tokyo, on September 28, 2011. The world largest underground discharge channel at 50 meters below ground is more than 6 km long, and can hold 670,000 tons of water at maximum, the management office said. The ceiling of the concrete water tank is supported by 59 pillars, each 7 meters long, 2 meters wide, 18 meters tall, and weighing 500 tons.
A man stands outside a tunnel opened by unsuccessful treasure hunters near the northern Greek town of Grevena, on December 12, 2011. As years of austerity take an ever harsher toll, more and more Greeks are finding solace in tales of buried riches dating from the near-bankrupt country's turbulent recent past. Despite rife urban legends of rich findings, prospective treasure hunters usually end up in police cells.
(AP Photo/Nikolas Giakoumidis)
Tourists walk towards the main entrance of the Niah Great Cave at Niah National Park, in the Malaysian state of Sarawak in Borneo, on March 29, 2012. Niah Caves contains the oldest remains of Homo sapiens found in Borneo, and features the world's largest limestone cave entrance as well as ancient rock paintings. Studies published recently have shown evidence of the first human activity at the Niah caves from ca. 46,000 to ca. 34,000 years ago.
Speleologist Carlos Lopez looks at paintings of seals believed to be the world's oldest works of art -- approximately 42,000 years old, in the caves of Nerja, southern Spain, on February 14, 2012. These seal paintings are the only known artistic images created by Neanderthal man, according to scientists.
(Jorge Guerrero/AFP/Getty Images)
A visitor looks into a tunnel forming part of the old Malta Railway which was closed down in 1931 in Floriana, outside Valletta, on October 23, 2011. Locals and tourists flocked to see part of the network of tunnels which were opened to the public on Sunday for the first time in a generation, according to local media.
(Reuters/Darrin Zammit Lupi)
Israelis participate in a speed dating event on the eve of Tu Be'Av, the Jewish holiday of love, in Zedekiah's Cave, also known as Solomon's Quarries, under Jerusalem's Old City, on August 14, 2011. Hoping the exotic setting would provide a conducive backdrop for romance, organizers brought 70 Israeli singles to the subterranean quarry for the unique speed-dating marathon to mark the 15th day of the Hebrew month of Av, the Jewish holiday of love.
(AP Photo/Sebastian Scheiner)
A hotel staff member guides visitors inside a bomb shelter under the garden of the Metropole hotel in Hanoi, Vietnam, on May 21, 2012. The Metropole hotel opened the formerly-functional underground bomb shelter for visitors. U.S. folk singer Joan Baez and actress Jane Fonda, foreign war correspondents and foreign diplomats took shelter in it during the Christmas Bombings in 1972, as well as other bombings during the Vietnam war.
An employee of French National Radioactive Waste Management Agency ANDRA works at the digging of a tunnel 500 meters underground at the Underground Research Laboratory of the Agency in Bure, Eastern France, on June 11, 2012. Bure, about a three hour drive from the French capital, is currently home to an underground laboratory, a precursor to the waste site. France and Finland are close to approving the world's first permanent radioactive waste storage sites.
A family, displaced by the recent fighting between forces loyal to Yemen's outgoing President Ali Abdullah Saleh and tribal militants loyal to anti-regime protesters, sits in a cave in the mountainous area of Arhab, north of Sanaa, on January 8, 2012, as they wait to receive Qatari aid.
Workers prepare to take a load of pipes and other material on a rail car into the northbound Sound Transit light rail tunnel between Seattle's Capitol Hill neighborhood and the University of Washington, on October 12, 2011, in Seattle. The tunnel is part of a three-mile link between downtown and the University of Washington that is scheduled to open in 2016.
(AP Photo/Ted S. Warren)