There's much left unexplored beneath Earth's surface, including miles of underground rivers. What forms them? What lives there? And how do these subterranean waterways help science?
Off the midwestern coast of the Philippines sits Sabang, a small tropical beach village surrounded by jungle. From Sabang, a fifteen-minute boat ride or a short hike through the mountains will take you to the Puerto Princesa Subterranean River National Park. There, nestled amongst the mangrove forests and freshwater swamps, you’ll find the entrance to one of the longest underground rivers in the world.
More intrepid travelers can venture another 3 kilometers—with a special permit, that is. But after that, the subterranean river remains unnavigable and yet-to-be explored.
The river starts in the nearby mountains but soon appears to vanish as it shifts course to flow beneath the Earth’s surface for a total of 8.2 kilometers (or about 5 miles). An average tourist can easily explore the first 1.5 kilometers of the river and the cave system it has helped to carve out via canoe and led by a guide with a flashlight. More intrepid travelers can venture another 3 kilometers—with a special permit, that is—to see more of the impressive cave-dwelling stalagmites and stalactites up close. But after that, the subterranean river remains unnavigable and yet-to-be explored.
Even amongst underground rivers, the Puerto Princesa is unique. At its end, it flows directly into the sea, which leaves the end of the river subject to the influence of ocean tides. UNESCO has named the river a World Heritage Site and to protect it, only allows 600 people to visit per day. Such a unique habitat is also home to a unique and thriving range of flora and fauna including 800 plant species and animals like crabs, snakes, and bats that are found nowhere else on the planet. In 2017, a new species of Huntsman spider was found wedged in the cracks of the cave walls along the underground riverbanks of the Puerto Princesa. I can tell you from my experiences with surface-dwelling Huntsman spiders in Australia that they are large and move fast, but beyond scaring you out of your wits, they're relatively harmless.
The Sistema Sac Actun, as the underground network is called using the Yucatec Mayan word for 'white cave,' stretches for 95 miles.
The Puerto Princesa was thought to be the longest underground river until 2007 when two underground rivers and cave systems in Mexico were found to be linked. Two divers, Steve Bogaerts and Robbie Schmittner, had been exploring the underground cave systems buried beneath the Yucatan Peninsula for years and were convinced they had to be connected. After more than 500 dives of several hours each, they finally set out one morning from two separate points in the cave system and successfully found each other at the long-sought-after connection point in the middle. The Sistema Sac Actun, as the underground network is called using the Yucatec Mayan word for “white cave,” stretches for 95 miles.
The longest underground river in the United States is thought to be the Lost River in Indiana which disappears beneath the surface shortly after it begins. The entire river is around 87 miles long and its course diverts underground for about 23 of those miles leaving a dry river bed at the surface. Its cave system has not been fully explored, however, so its underground portion could extend for hundreds of miles before eventually emptying into the White River.
How are underground rivers formed?
Underground rivers like the Puerto Princesa and the Lost River are carved out of karst, a type of porous limestone rock. Rainwater from the Earth’s surface finds its way into tiny cracks in the rock and percolates or slowly drips down until its gravity-powered descent is stopped by a more solid layer of rock. As more and more water collects, the pressure that water puts on the surrounding rock increases. Rainwater also has some level of acidity, so the water begins to carve its own path through the rock.
Natural sinkholes in the limestone can create caverns for the underground river to flow through. When water levels rise, the water can then erode the ceiling of a cave, sometimes so much that when the water recedes again, the cave collapses. Such a collapse is thought to have created the Reka River Gorge in Slovenia, where the once underground Reka River now sees daylight from the bottom of a 3-kilometer-long gorge 100 meters below the surface.
Science and underground rivers
As an astronomer, I'd be remiss if I didn’t also point out that karst sinkholes are very useful for building radio telescopes. Collecting light at longer radio wavelengths requires a large collecting dish—so large that it can be hard to build such a dish that is structurally sound to sit above ground. Instead, the two largest radio dishes in the world—the Arecibo Telescope in Puerto Rico and the Five-hundred-meter Aperture Spherical radio Telescope (or FAST) in the Guizhou Province in China—take advantage of already existing karst sinkholes.
Fluorescent dyes are useful for tracing underground rivers through dark caves.
The study of the history, structure, and make up of these caves and underground karst features is a science known as speleology. Dye tracing is one tool speleologists can use to track the path of an underground river. When added to the water, the presence of the dye found downstream can mark the river’s progress and likely path. The amount of dye measured downstream can further indicate the volume of water that must flow in the river as revealed by how much the dye gets diluted. Fluorescent dyes, in particular, are useful for tracing underground rivers through dark caves. Interestingly, a similar kind of dye tracing is also used to detect leaks in plumbing and even in medicine to analyze blood circulation in the body.
Manmade underground rivers
Not all underground rivers are created by nature. Some are the result of humans paving and building over rivers that previously saw the light of day. Below the streets of Sydney, Australia, runs a river that was diverted into holding tanks in the 1700s before it was drained in 1850 because it had become too polluted. The Neglinnaya River used to form a moat around the Kremlin, but its frequent flooding was a hindrance to construction plans. The river was diverted underground before becoming so polluted with industrial waste from the construction that it was merged with the sewer system. A similar story can be told for the Minetta Brook, which runs beneath New York City. These former freshwater rivers that once flowed above ground now create underground labyrinths of grime and waste beneath the surface.
These former freshwater rivers that once flowed above ground now create underground labyrinths of grime and waste beneath the surface.
Our explorations of the surface of the Earth extend upward 29,029 feet, as marked by the tip of Mount Everest, the highest point above sea level. Because the Earth is not a perfect sphere and instead bulges a bit around the waist, we can actually get a little further from the Earth’s center at the top of Mount Chimborazo at a distance just shy of 36,000 feet. But that’s less than 0.2% of the Earth’s radius! There is a whole lot more to explore beneath the surface.