The oldest part of Mammoth Cave in Kentucky was formed 10 million years ago, 9.5 million before Homo sapiens made an appearance. From the National Geographic book Journeys of a LifetimeUnderground City, Montreal, CanadaTake one of the 120 entrances into the largest man-made underground network in the world. Grab your gear: These are the best places to hit the slopes and relax in front of a blazing fire.
The results were officially presented at the Hidden Earth national caving conference last month.
Take a 3-D tour of the Chinese supercave here, and read more about the Miao Room from National Geographic here! Astronaut "Selfie": The First Instagram From Space (Video)The first Instagram from Space was unsurprisingly a selfie. NASA Television Produces Live HD Video of Earth from Space (Live Video Link)NASA recently launched the HDEV experiment, putting live-stream cameras on the International Space Station. Underground you can still be the first person on Earth to take the small step, or the giant leap.
This story is part of a special National Geographic News series on global water issues.Rivers are the lifeblood of many plant, animal, and human communities. Tibbetts Brook starts just north of the Bronx and then flows into the borough's leafy Van Cortlandt Park, where it fills a small lake. A tributary of the mighty Hudson River, the Sawmill River flows about 23 miles (37 kilometers) from Chappaqua, New York, to Yonkers, just north of New York City.
The Wein River was driven below Vienna long ago, where it was integrated into the city's sewer system. Beautiful arched foundations can be seen along the underground Bradford Beck River in Bradford, England.
This large area is the end of the tunnel that contains the River Sheaf as it travels through the city of Sheffield.
A number of watercourses through London were buried over the centuries, including the River Westbourne, which flowed from Hampstead through Hyde Park to Sloane Square and into the River Thames at Chelsea.The River Westbourne had been an important source of drinking water for the growing city, and conduits were built for that purpose in 1437 and 1439.
The largest of London's lost rivers, the River Fleet flows four miles (6.4 kilometers) underground through downtown before it enters into the Thames. The Roman settlement of Londinium, the precursor to today's city, was bisected by a river called the Walbrook, perhaps named because it flowed under the fortified walls. Sandra is a leading authority on international freshwater issues and is spearheading our global freshwater efforts. For more than 15 years, Osvel Hinojosa Huerta has been resurrecting Mexico's Colorado River Delta wetlands. The National Geographic Society aims to be an international leader for global conservation and environmental sustainability.

The Saw Mill River in Yonkers, New York, is being exposed to fresh air after decades of burial. Uncovering buried streams has had huge impacts in places as diverse as Seattle, Washington, Kalamazoo, Michigan, and even Seoul, Koreaa€”improving local water quality, providing habitat for fish and birds, and turning neglected parking lots and roads into public parks that boost neighbors' property values and can revitalize entire cities.
Often, engineers found that the simplest solution was to bury the streams, routing the water into pipes and paving over the top. Yet many of the world's rivers have been dammed, degraded, polluted, and overdrawn at alarming rates.Some of the world's great rivers, from the Colorado to the Indus, don't always reach their ends because people have diverted so much water for agriculture, industry, and municipal uses. Since the early 1900s, the last 2,000 feet (600 meters) of the river have been entombed in a flume underneath downtown Yonkers.The river was covered gradually. Army Corps of Engineers diverted the Park River beneath Hartford, Connecticut, in what was one of the largest and most expensive projects the Corps had tackled up to that point. Today it drains into the Moskva River via two openings.The Kremlin was built on a hill west of the Neglinnaya River, with a moat filled from its channel. Urban explorer and photographer Steve Duncan believes this section underlies Bradford City Hall, a Victorian-era structure built in the 1880s.
However, by the 19th century, the water had become too filthy to use for anything except a sewer.In the early part of the 19th century, the River Westbourne was channeled into underground pipes to make more room for the growing areas of Chelsea, Paddington, and Belgravia.
The stream started in what is now Finsbury and flowed into the River Thames near today's Cannon Street Railway Bridge.The river was long used as a sewer, as well as for drinking water, and its quality began to decline as early as the Roman period.
For every pledge, Change the Course will restore 1,000 gallons back to the Colorado River. When individuals pledge to use less water in their own lives, our partners carry out restoration work in the Colorado River Basin.
In addition to exposing a waterway that had long been covered, the effort has already sparked plans for a new minor-league ballpark and new housing. In many cases, city residents don't even know that there are buried waterways under their feet. Well, to put it into perspective, the cave chamber is nine times the size of Houston’s astrodome.
Other rivers have been completely covered over by development, as people attempted to "tame" nature by ending flooding and maximizing usable land area.But what happens to once-thriving freshwater ecosystems when the rivers they depend on are entombed in sewer pipes beneath layers of concrete and soil? The Lenape called the stream Mosholu, meaning "smooth or small stones."In the 1690s, property owner Jacobus Van Cortlandt built the small dam that created the lake, in order to power a sawmill and gristmill.
Over time residents grew weary of the river's flooding, so in 1792, the city diverted its course into a new canal, and filled in the original bed. Paul's Cathedral.The Fleet was forced underground and combined with the sewer system in the 19th century. By 1598, historian John Stow wrote that the Walbrook was being paved over with bricks to make way for housing.By the 1830s, what remained of the Walbrook's flow was coursing through city sewer lines, where it remains today.

Peace reigns in this surreal world of spectacular rock formations, stalactites, stalagmites, fossils, and fruit bats lying beneath untouched rain forest. Over time, the bridges got bigger and closer together, as Yonkers experienced rapid industrialization. John Kulick of Huck Finn Adventures, who has guided float trips through the subterranean section, told the New York Times he has seen eels, carp, and stripers in the dark water.
You couldn't build on top of them, and the rapidly growing populations in many cities were throwing all their sewage into open water. Army Corps of Engineers says there are no records of the agency covering streams in Yonkers, and said the Corps would not have had jurisdiction to do so. Ironically, it was often rivers and streams that attracted people in the first place, but those very sources of life can fall victim to the expanding concrete jungle.This was the case in the late 1800s for Sunswick Creek in the Queens section of New York City. The city of New York acquired the land that became Van Cortlandt Park in 1888.Today only a small part of Tibbetts Brook remains above ground.
Eventually, the river was completely covered over, and it has historically suffered from pollution and intense development.In 2007, a $34 million plan was approved to expose, or "daylight" much of the buried section of the Sawmill River, as part of a $3 billion redevelopment project for Yonkers. By the 13th century, the Fleet had already become polluted, since it was long used as a sewer. During the Vietnam War (1954-75) thousands of Viet Cong were based in the 125-mile (200-kilometer) network of tunnels.
This will include a new city park.Scientists hope that exposing the river to daylight will help restore it to a better state of health. Much of its water was also gradually diverted for industry.After the Great Fire of London in 1666, the river was reshaped into a canal. Over time this was increasingly covered over by development, until the process completed around 1870.Today the River Fleet can still be heard gurgling below a grating at Ray Street. Participants counted eight species of fish, though more than 20 were counted by scientists in the past.
Nine centuries of mining have produced miles of subterranean passages and gigantic caverns to a depth of 440 feet (134 meters). The bunker could operate for 14 days, after which you would be cast out into the radiation-saturated debris of Berlin after a nuclear attack.
See 457 feet (139 meters) of sewer at work (with attractions including a manual flusher trolley that keeps detritus on the move) and learn about waste disposal through the ages. Inside the awesome inner sanctum is a sarcophagus, built of granite from Aswan, 625 miles (1,006 kilometers) away.

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