There are still some hidden places on Earth that are unknown and unexplored. Every year, intrepid cavers discover new underground passageways and chambers, extending the dimensions of the world’s longest and largest cave systems. But incredibly, there are still untold miles of caves waiting to be found.

“We’ve known the tallest mountains for hundreds of years and located the deepest spots of the oceans four or five decades ago, but there's no technology that will tell us where the deep caves are,” says Michael Ray Taylor, a journalist and author who has explored many of the world’s biggest caves. “There’s a saying among cavers: ‘Caves are where you find them.’”

In the 40 years that Taylor has been writing about caving, he’s seen amazing growth in the discovery and mapping of really big caves, especially outside of the United States. Some of the really large caves in China and Eastern Europe only began to be seriously explored and mapped a few decades ago, and one of them may eventually overtake Mammoth Cave in Kentucky as the world’s longest.

Caves can form through a number of geological processes including volcanic activity (lava tubes) and erosion (sea caves), but nearly all of the world’s longest caves are so-called “solution” caves. Solution caves are formed when mildly acidic surface water seeps into cracks and fissures within a subterranean layer of limestone. The acidic solution eats away at the limestone, creating ever-widening openings that become caves.

Caves can also form from the bottom up. In a “hypogene” cave, water bubbles upward from deep within the earth, often as a byproduct of volcanic activity. The water is rich in sulfur and carbon dioxide, forming even stronger acids that eat away at the limestone above, creating caves. Several of the world’s longest caves were formed this way and took longer to discover because they had few, if any, surface entrances.

As the sport of caving has grown, park systems in the United States and elsewhere have begun to offer “wild caving” tours to adventurous visitors who want to go beyond the well-trod, well-lit paths.

“Anyone who’s brave enough to see what it’s really like to crawl through a tight spot can now take a safe, guided tour where all of the equipment is provided,” says Taylor, author of Hidden Nature: Wild Southern Caves. “And you get a true experience of what it's like to explore a wild cave.”

Here are five of the longest cave systems in the world as of 2024:

Mammoth Cave in Kentucky, USA, is by far the longest continuous cave system on record. Humans first entered Mammoth Cave between 4,000 and 5,000 years ago, likely to mine the cave walls for gypsum and other minerals. Indigenous explorers pushed nearly 20 miles into the cave relying solely on torch light.

By the early 19th century, Mammoth Cave was already a popular tourist attraction known for its impressive stalactites and cathedral-sized chambers. In 1842, an enslaved cave guide named Stephen Bishop drew the first map of Mammoth Cave’s underground labyrinths from memory. But it wasn’t until the 1950s that cavers attempted to map the full extent of the gigantic cave.

A major breakthrough came on September 9, 1972, when a team of explorers discovered a long-sought passageway between Mammoth Cave and the nearby Flint Ridge Cave System, which was the longest cave in the world at the time. The newfound connection more than doubled the length of Mammoth Cave to 144 miles, but that discovery was only the beginning.

“Mapping a really long cave is a group effort that sometimes lasts generations of cavers,” says Taylor. “People have been mapping Mammoth Cave for over 200 years now, and they're still finding new passages.”

Mammoth Cave remains a hugely popular tourist attraction, drawing more than half a million visitors a year.

The Yucatán Peninsula in Mexico is home to dazzling geological formations called cenotes, large sinkholes that are filled with crystal-blue water. The cenotes were formed by the same limestone solution process as the caves in Kentucky. But about 18,000 years ago, sea levels rose and transformed the underground passageways into the world’s largest underwater cave systems.

The cenotes near the ancient Mayan city of Tulum are popular tourist attractions for swimming or shallow scuba diving. They are also entry points into massive underwater cave systems that are only accessible to trained and certified cave divers.

“The caves themselves are delightful to swim in, because the water is a perfect temperature, not icy like some sea caves,” says Taylor. “But that’s also their danger, because if you’re not careful, they’ll lure you in beyond your skill level.”

The largest of the Yucatán underwater caves is Ox Bel Ha (Mayan for “Three Paths of Water”), which snakes for 308 miles beneath reefs, jungles and marshes, and is accessible by dozens of interconnected cenotes. The second-largest underwater cave system is nearby Sac Actun (“White Cave”), which currently measures 234 miles.

Both Ox Bel Ha and Sac Actun were once dry caves, so they developed features like stalactites, stalagmites and other “speleothems”—unique mineral deposits that only form in limestone caves.

“As the water levels rose, they're still down there,” says Taylor. “And so a lot of those passages are quite beautiful.”

The Guizhou Province in Southern China is known as the “Kingdom of Caves.” When Taylor first visited the region in the 1980s, he immediately recognized it as a classic “karst” landscape, where geological features like towers, sinkholes and caves have been carved out by millions of years of solutional erosion.

“The karst towers of Southern China are full of caves and the valleys beneath them are full of caves, too,” says Taylor. “I said at the time, they’re going to be mapping new mammoth caves here very soon, and that’s exactly what’s happened.”

The first entrance to Shuanghe Cave was discovered in the late 1980s by a local Guizhou farmer. Today, more than 259 milesof Shuanghe Cave have been mapped, making it the longest cave in Asia. Explorers have found 115 separate entrances to the massive cave system, which includes five underground rivers, several waterfalls and a few truly colossal chambers.

“There are some big, big passages,” says Taylor. “I mapped a river cave there once, and if you found the right pilot, you could have easily flown a helicopter in the entrance and then a half mile down and back again without touching the walls.”

Shuanghe Cave is also home to some truly unique cave-dwelling animal species (known as “troglobites”) like blind fish and red-spotted toads with teeth. Scientists have also recovered dozens of fossilized remains of giant pandas deep inside the cave, some as old as 100,000 years.

Jewel Cave and Wind Cave are located just 30 miles apart in the spectacular Black Hills region of South Dakota, USA. They’re the longest hypogene caves on Earth, formed from below by sulfur-rich, acidic water rising up from deep in the planet’s crust.

Jewel Cave gets its name from the abundance of sparkling calcite crystals that surround the walls of the cave’s only natural entrance.

“Two miners filed a mining claim, because they thought they had found a cave full of diamonds,” says Taylor. “It turned out that their glistening beauty is from the water that permeates them. And once they dry up, they don't look very nice at all.”

Portions of nearby Wind Cave formed 300 million years ago, making it one of the oldest caves on the planet. Both Jewel Cave and Wind Cave showcase remarkable mineral deposits (speleothems), including delicate, coral-like formations called “cave popcorn,” “gypsum flowers” and “helictite bushes.”

Wind Cave contains more “boxwork” than all other caves on Earth combined. The striking geometric shapes on the cave’s ceilings were formed inside the cracks of gypsum deposits that have long since eroded away.

“One thing about exploring and mapping these big caves—if you weren’t a conservationist before you started, you become one,” says Taylor. “Other people need to see these places and so we need to protect them.”

According to air pressure studies, only 5 to 10 percent of Jewel Cave and Wind Cave have been mapped, meaning that potentially thousands of additional miles of passageways and chambers remain to be found.

At 164.4 miles, the Optymistychna Cave in Ukraine is officially the longest cave in Europe and the largest gypsum cave in the world.

Optymistychna Cave wasn’t discovered until 1966 and remained largely sealed off to Western cavers until the collapse of the Soviet Union in the early 1990s. Only in recent decades have cavers begun to map the depths of this Gypsum Giant, a rare type of cave formed in a 60-foot horizontal layer of gypsum beneath a cap of limestone.

The narrow passageways of the Optymistychna (“Optimistic" in Ukrainian) Cave are lined with sparkling gypsum deposits, and the cave also contains several underground lakes, including one that’s nearly 900 square feet and 20 feet deep.

Unlike the largest American caves located in national parks, the Optymistychna Cave hasn’t been barraged by tourists. Adventurous travelers can still explore the impressive cave in its wild state through local tour companies.