Mammoth Cave hides 685.6 km of mapped passage beneath a single Kentucky ridge — roughly the road distance from London to Edinburgh — and surveyors are still adding to it. The fifteen systems below run from 147.9 km up to that figure and sit under eight countries on four continents, ranked strictly by surveyed length: the passage cavers have actually measured and tied into one connected system. That rule matters more than it sounds. A cave does not grow because someone guesses at it; it grows when a survey is pushed further or a connection is finally proven, which is how Sistema Sac Actun absorbed neighbouring Dos Ojos in 2018, and how Mammoth took the top spot in 1972. Every cave here exists today and is still being surveyed. Very few can simply be walked into: a handful run guided tours, most demand serious caving experience, and Lechuguilla is sealed to all but researchers.
15 Longest Caves in the World (2026)
The record The longest cave in the world is the Mammoth Cave System, in the United States — 685.6 km (426.0 mi), surveyed length. Source: Geo2 World Long Caves; Wikipedia (686 km). Verified .
Longest Caves in the World The Ranking at a Glance
Fun Facts About the World's Longest Caves
- One connection can rewrite the whole list. Sistema Sac Actun grew by more than a third in 2018 when divers proved a link to the neighbouring Dos Ojos system, and the smaller name simply vanished into the larger one.
- The first cave to pass 100 km was Swiss, not American. Hölloch crossed that mark by 1968 and was widely considered the world's largest cave until the Flint Ridge–Mammoth connection was proven in 1972.
- Roughly 95 percent of all the boxwork ever found is in one cave. Wind Cave holds almost the entire world supply of these honeycomb calcite fins, which are left standing when the rock around them dissolves away.
- One of these caves was carved from below, not above. Lechuguilla Cave was dissolved by sulfuric acid formed when hydrogen sulfide rose from oil and gas deposits — the reason it is lined with gypsum instead of ordinary flowstone.
- Europe's longest cave is made of gypsum, not limestone. Optymistychna Cave spreads sideways into a shallow maze rather than descending, because gypsum dissolves far more readily than the rock most caves form in.
- A cave on this list is also a major archaeological site. A deep pit in Sistema Sac Actun called Hoyo Negro has produced ice-age animal bones alongside a young woman known as Naia, among the earliest human remains found in the Americas.
1. Mammoth Cave System
At 685.6 km of surveyed passage, Mammoth Cave System is the longest known cave on Earth by a margin of roughly 144 km — no other cave is close. It runs beneath the sandstone-capped ridges of central Kentucky, where a hard caprock has protected a limestone layer long enough for water to carve level upon level of galleries, domes and river passage. The scale is not a single grand tunnel but an accumulation: hundreds of kilometres of interlocking corridors that surveyors have spent two centuries stitching together.
It reached the top of this list in 1972, when cavers proved a link between the neighbouring Flint Ridge system and Mammoth itself and the two became one. The cave sits inside Mammoth Cave National Park, a UNESCO World Heritage Site, and is among the few entries here that ordinary visitors can see: ranger-guided routes run through the historic entrance and past landmarks used by nineteenth-century tour parties. Beyond the lit sections, exploration continues, and the surveyed total still creeps upward.
2. Sistema Ox Bel Ha
Sistema Ox Bel Ha reaches 541.7 km, and almost every metre of it is underwater — making it both the second-longest cave in the world and comfortably the longest flooded one. It lies beneath the jungle south of Tulum on Mexico's Yucatán Peninsula, where porous limestone has dissolved into a vast horizontal network carrying fresh groundwater toward the Caribbean. Its Mayan name translates as "three paths of water", and the cave is reached not by walking but by descending through cenotes, the collapse sinkholes that puncture the forest floor.
Surveying it is diving work, done on long penetrations by teams from CINDAQ and the Quintana Roo Speleological Survey, who map passage a reel at a time and add new kilometres most years. The water is famously clear and layered: divers pass through a halocline where fresh water floats on salt, a shimmering boundary that blurs the view like heat haze. That combination of scale, clarity and sheer difficulty makes Ox Bel Ha one of the most demanding survey projects anywhere underground.
3. Shuanghedong Cave Network
Shuanghedong Cave Network totals 440.9 km, which makes it the longest cave in Asia and the largest dolomite cave system known anywhere. It threads through the hills of Suiyang County in Guizhou, a province whose thick soluble rock and heavy subtropical rainfall have produced some of the most spectacular karst on the planet. Unusually, the network opens to the surface through more than a hundred separate entrances and drops some 912 m from its highest point to its lowest.
Most of what is known about it comes from a long-running series of international expeditions that began in 1988, pairing Chinese speleologists with teams from France, Italy, Portugal and Belgium. Each season pushes new leads and re-surveys old ones, and the figure has climbed steadily as a result — the twenty-fourth expedition reported its latest additions in late 2025. Because dolomite behaves differently from ordinary limestone, the passages here also give geologists a rare view of how a cave forms in harder, slower-dissolving rock.
4. Sistema Sac Actun
Sistema Sac Actun runs to 386.1 km, the second of the two enormous flooded systems beneath Tulum and the second-longest underwater cave in the world. Like its neighbour Ox Bel Ha it is a maze of low, wide, water-filled galleries in young limestone, linked to the surface by cenotes and fed by rain that percolates down through the forest floor. Divers navigate it on permanent guidelines, and much of the passage sits only a few metres below the water table.
Its current size dates from 2018, when the Gran Acuífero Maya project proved a connection between Sac Actun and the well-known Dos Ojos system, and the smaller name was absorbed into the larger one overnight. The cave is also an archaeological site of the first order: a deep chamber called Hoyo Negro has yielded the bones of ice-age megafauna alongside a young woman known as Naia, whose remains count among the earliest evidence of people in the Americas.
5. Jewel Cave
Jewel Cave holds 354.6 km of mapped passage, ranking it fifth in the world and second in the United States. It sits in the ponderosa pine country of the Black Hills in South Dakota, and takes its name from what early explorers found lining its walls: a dense crust of calcite crystals that catches lamplight like a geode turned inside out. The passages are typically narrow and angular, following fractures in the limestone rather than the broad river tunnels of a cave like Mammoth.
The cave is protected as Jewel Cave National Monument, and parts of it are open on ranger-led tours that descend by lift into the mapped sections. What makes it genuinely intriguing is the airflow: the volume of air that moves in and out of the known passages implies a far larger system still waiting beyond the current survey limit. Cavers have been pushing that frontier for decades, and the total has grown by hundreds of kilometres since the monument was established.
6. Wind Cave
Wind Cave contains 270.5 km of surveyed passage packed into a remarkably small footprint, making it one of the densest cave mazes known — sixth-longest in the world, yet folded beneath just a few square kilometres of South Dakota prairie. It is named for the air that rushes through its natural entrance as outside pressure rises and falls, a breathing effect strong enough to be audible and the clue that first drew explorers underground in the 1880s.
Its real distinction is decorative. Wind Cave holds an estimated 95 percent of all the boxwork ever discovered — delicate honeycomb fins of calcite that project from the walls and ceilings like the cells of a wasp nest, formed when the surrounding rock dissolved away and left the veins behind. Above ground, the park protects mixed-grass prairie grazed by bison and elk, and in 1903 it became the first cave anywhere in the world to be given national park protection.
7. Clearwater Cave System
The Clearwater Cave System measures 265.9 km, the longest cave in Malaysia and the centrepiece of the extraordinary karst beneath Gunung Mulu National Park in Sarawak. Its trunk passages are enormous — a subterranean river runs through galleries wide enough to swallow a village — and the surrounding park, a UNESCO World Heritage Site, contains some of the largest cave chambers and most impressive limestone pinnacles on Earth.
Serious exploration began with a Royal Geographical Society expedition in 1977 and 1978, which surveyed the first 24 km and revealed how much more there was to find. Expeditions have returned regularly ever since, connecting adjacent caves into the single system recognised today. Sections near the entrance are reachable on boardwalk and boat as part of the park's visitor routes, but the great majority of the network lies well beyond, in river passage that only expedition teams reach.
8. Optymistychna Cave
Optymistychna Cave threads 264.6 km through a bed of gypsum in western Ukraine — the longest cave in Europe and the longest gypsum cave anywhere in the world. It sits near the village of Korolivka in Ternopil Oblast, and it is shaped quite unlike the others on this list: instead of descending, it spreads sideways into a dense two-dimensional labyrinth squeezed between layers of rock, with hundreds of kilometres of passage contained within a shallow vertical range.
Gypsum dissolves far more readily than limestone, which is why such a maze could form here at all, and the walls carry crystal growths that glitter under torchlight. The cave was entered in 1966 by speleologists from Lviv, whose optimism about what lay beyond the first squeeze gave it its name, and Ukrainian caving clubs mapped it steadily across the Soviet-era decades that followed. Surveying has slowed considerably in recent years, but the cave itself remains intact underground.
9. Lechuguilla Cave
Lechuguilla Cave has 244.8 km of surveyed passage and drops 484 m below the New Mexico desert, but its fame rests on how it formed rather than how far it runs. Where most caves are dissolved from above by mildly acidic rainwater, Lechuguilla was eaten out from below: hydrogen sulfide rising from oil and gas deposits mixed with oxygenated groundwater to make sulfuric acid, which carved the rock and left behind gypsum in extravagant quantities.
The result is a cave of exceptional mineral decoration — gypsum chandeliers several metres long, hair-fine crystal filaments, delicate soda straws and rafts of cave pearls, much of it untouched since it grew. Cavers broke into the modern system in 1986 by digging through rubble at the bottom of a small historic pit inside Carlsbad Caverns National Park. Access is restricted to approved researchers and survey teams; there are no tourist routes, and there is no plan to build any.
10. Sistema del Alto Tejuelo
Sistema del Alto Tejuelo has been surveyed to 220.5 km, making it the longest cave in Spain and one of the great prizes of European speleology. It runs beneath the Porracolina massif in Cantabria, in the limestone uplands between the Miera and Asón valleys, and takes its name from a 950 m peak sitting roughly over the middle of the network. Around twenty separate entrances give access to it, and the system spans some 626 m from top to bottom.
Alpine caves like this one are assembled rather than discovered: teams work shafts and streamways for years, then a single squeeze links two known caves and the map doubles overnight. Alto Tejuelo has grown exactly that way, most dramatically in early 2024 when a new connection pushed it past its Cantabrian rivals. The region's cavers continue to hunt for the junctions that would extend it further, and the surveyed figure has climbed again since.
11. Fisher Ridge Cave System
Fisher Ridge Cave System runs to 215.9 km beneath Hart County, Kentucky — and it has the peculiar distinction of being a giant that sits almost next door to a bigger one. It occupies the same band of cave-bearing limestone as Mammoth, only a short distance to the east, and consists of two substantial halves joined by a crawl some 300 m long, one side dominated by large dry tunnels and the other by active streams.
Its discovery was recent and unusually abrupt: in January 1981 a group of Michigan cavers dug into a sinkhole and broke straight into open passage that kept going. Four decades of survey trips have followed. The obvious question — whether Fisher Ridge and Mammoth will one day be shown to connect, creating a single system of almost inconceivable length — has driven a great deal of that work, and remains unanswered. The cave is not open to visitors.
12. Hölloch
Hölloch reaches 209.6 km and descends 1,033 m into the mountains above Muotathal in the Swiss canton of Schwyz, making it the longest cave in Switzerland. Its name means "hell hole", and the character of the place earns it: this is a steeply inclined alpine system of tight rifts, climbs and river passages, prone to sudden and dangerous flooding when snowmelt or heavy rain pours into the mountain above.
It has a special place in caving history. By 1968 Hölloch had become the first cave in the world with more than 100 km of surveyed passage, and until the Flint Ridge–Mammoth connection was proven in 1972 it was widely regarded as the largest known cave system on Earth. A short section near the entrance is fitted out for guided visits, and longer expedition-style trips run into the deeper cave, but the bulk of the system stays the preserve of experienced cavers working around the flood seasons.
13. Siebenhengste-Hohgant-Höhle
The Siebenhengste-Hohgant system totals 164.5 km and plunges 1,340 m, which makes it Switzerland's second-longest cave and one of the deepest on this list. It lies in the Bernese Oberland north of Lake Thun, between the villages of Habkern and Eriz and within sight of the Interlaken peaks, riddling a block of alpine limestone that is drained from top to bottom by water heading for the lake.
More than forty entrances puncture the mountainside, many of them shafts opening straight into the hillside at different altitudes — the reason the system has such enormous vertical range. Exploration began in 1966 with cavers from the Club Jurassien, and the current preoccupation is the search for a link to the Bärenschacht below, a connection that would add well over 80 km at a stroke and reshape the Swiss rankings entirely. For now the two remain tantalisingly separate.
14. Schönberg-Höhlensystem
The Schönberg-Höhlensystem has been surveyed to 156.9 km, making it the longest cave in Austria and the longest in the Eastern Alps. It lies within the Totes Gebirge — the "Dead Mountains" — a bleak, largely waterless limestone plateau where rain and snowmelt vanish underground almost immediately, leaving a bare karst surface above and an immense drainage network below. The system reaches 1,061 m deep beneath the Schönberg peak.
Caves in this plateau are explored on long summer expeditions from surface camps, with teams working shafts on the high ground and following the water downward toward resurgences in the valleys. Progress is slow and hard-won, and the system has been assembled over decades as separate caves were pushed into one another. The ice found in some upper sections is a reminder of how cold air pools in these entrances, preserving frozen deposits year-round well below the snowline.
15. Sistema del Mortillano
Sistema del Mortillano closes the list at 147.9 km, and it is the second of Cantabria's two entries — a reminder that this small stretch of northern Spain is among the most cave-rich regions in the world. The system sits above the Asón valley, reaches roughly 950 m in depth, and is drained by an unusual abundance of water: at least seven distinct underground rivers run through its galleries, each with its own tributaries.
Those rivers are what shaped it. Rather than one master conduit, Mortillano is a braid of separate streamways stacked and interleaved through the massif, which is precisely what makes surveying it such a long undertaking. Exploration began in the 1960s, lapsed, and was taken up again in the 1990s by teams who have since added tens of kilometres and found more than twenty ways in. Like most caves on this list it is wild — no lights, no walkways, and no tours.
FAQ: The World's Longest Caves
What is the longest cave in the world?
Mammoth Cave System in Kentucky, with 685.6 km of surveyed passage — about 144 km more than the next cave on the list. It has held the record since 1972, when cavers proved that the neighbouring Flint Ridge system and Mammoth were one connected cave.
How is a cave's length actually measured?
By survey, not estimate. Cavers measure passage section by section with tape, compass and clinometer or a laser instrument, then tie those measurements into one connected map. Only passage that has been surveyed and proven to connect counts, which is why totals rise in steps rather than smoothly.
Can you visit the world's longest caves?
A few of them, in part. Mammoth Cave, Jewel Cave, Wind Cave, Hölloch and Clearwater all have sections open on guided routes. The rest are wild caves needing real experience and permission, and Lechuguilla is restricted entirely to approved researchers and survey teams.
What is the longest underwater cave in the world?
Sistema Ox Bel Ha beneath Tulum in Mexico, at 541.7 km. It is second overall on this list but first among flooded caves, and it is explored entirely by divers working from cenotes — the collapse sinkholes that open through the forest floor of the Yucatán Peninsula.
Why do cave rankings keep changing?
Because these caves are still being explored. Every season adds surveyed kilometres, and occasionally a single proven connection merges two large caves and reorders the table overnight. Several systems here are also being pushed toward neighbours that would add tens of kilometres if a link is ever found.
Planning more of your trip? Keep exploring more world record-breakers.