What Are Subterranean Rivers and How Do They Form?

Subterranean rivers are water bodies that flow through channels beneath the Earth's surface. In fact, you can't see them from above because they run deep underground through hidden networks.

These water systems come in two types: natural and artificial.

• Natural: ones form when rain soaks into the ground and moves through soil and rock, cutting paths through caves, cracks, and caverns as water dissolves rock over thousands of years.
• Artificial: humans create artificial subterranean rivers through urban building and mining, like the Mondmilch River (Moon Milk River) in the Alzola mines of Guipuzkoa which got its name from its white-colored water. These man-made waterways typically form when construction changes natural drainage patterns.

Subterranean rivers most commonly develop in karst areas where soft rocks like limestone let water carve paths below ground. As this water flows, it carries sediments, which are mostly from limestone breaking down, through the subterranean channels.

These rivers don't always stay hidden forever. They often resurface as springs, join surface waters, or empty into seas and oceans. Some that flow to coastal areas bring sand to beaches, while others provide water that people pump through wells.

Though rock layers naturally filter subterranean river water, pollution from surface activities can still seep in and contaminate these systems. The dark, nutrient-limited environment of these waterways creates homes for specialized life forms that have adapted to these unique conditions. Because of their hidden nature and difficult access, many subterranean river systems remain partially unmapped, requiring specialized cave diving techniques to fully document their extent.

Curious about how water shapes our landscape above ground? Explore our in-depth look at the powerful forces that carve and transform river channels over time.

Subterranean rivers begin with rain. When rainwater falls, some of it soaks into the ground instead of flowing over the surface. This rainwater isn't pure, since it absorbs carbon dioxide from the air and soil, making it slightly acidic.

This mildly acidic water is the key to creating subterranean rivers, especially in areas with limestone. Limestone dissolves when exposed to acidic water over hundreds or thousands of years. As the rock dissolves, tiny cracks widen into passages and eventually into the tunnels that carry subterranean rivers.

Areas with lots of limestone are called "karst landscapes." You can recognize these places by their sinkholes, disappearing streams, and caves. These features all signal that water is doing its slow work underground.

The water in subterranean rivers comes from the same water cycle that feeds surface rivers. Rain falls, soaks into the ground, and travels downward until it reaches less permeable rock. Then it begins to flow horizontally, following the path of least resistance.

Over time, these underground waterways grow larger as more rock dissolves. Small cracks become passages, passages become tunnels, and eventually, entire subterranean river networks form.

As subterranean rivers flow, they create different features:

• Cave systems: develop when subterranean rivers carve out large hollow spaces.
• Cenotes: form when the roof of a subterranean river or cave collapses, creating a sinkhole filled with water. Think of cenotes as windows into the subterranean river world.

The difference between an subterranean river and a cenote comes down to water movement. Rivers flow because they follow a sloped path. Cenotes are more like pools because they form in depressions where water collects.

The Yucatan Peninsula in Mexico shows these processes in action. This region sits on a thick limestone platform and receives plenty of rainfall. The Yucatan even features a "Cenote Ring", which is a circle of sinkholes about 180 kilometers across. Scientists believe this ring formed because of a massive asteroid that struck Earth about 66 million years ago (the same one linked to the dinosaurs' extinction).

Not all subterranean rivers are the same. They come in different forms depending on how much of them we can see and access.

Some flow completely hidden from view, buried beneath layers of rock and soil. You might walk directly above one and never know it's there unless you have special equipment to detect it.

Other are partially visible. These rivers might flow through cave systems with openings to the surface, allowing people to see sections of the river while other parts remain hidden underground. Some even alternate between flowing on the surface and disappearing underground multiple times along their course.

Examples of famous subterranean rivers around the world:

Subterranean rivers exist on every continent except Antarctica. Some of the most impressive include:

• Puerto Princesa Subterranean River National Park (Indonesia): this UNESCO World Heritage site features a river that flows directly into the sea through a massive cave system. Tourists can boat through its main passage, which is home to unique ecosystems including bats and swiftlets.
• Sac Actun System (Mexico): with over 350 kilometers of mapped passages, this is one of the longest underwater cave systems in the world. Maya people considered cenotes connected to this system as sacred entrances to the underworld.
• Lost River (Indiana, USA): this river disappears underground and flows through miles of limestone caves before resurfacing. The system includes one of the largest cave openings in the United States.
• Subterranean River of Križna Jama (Slovenia): this European river flows through a cave with 22 underground lakes, creating an otherworldly landscape.

Underground rivers are just one fascinating part of water's endless journey on Earth. Explore the complete path water takes from sky to sea and back again in our related guide.

People often confuse subterranean rivers with aquifers, but they are quite different:

• Flow pattern: subterranean rivers flow through defined channels, much like surface rivers. Aquifers are more like underground sponges—layers of permeable rock saturated with water that moves very slowly through tiny pores.
• Speed: water in subterranean rivers can flow relatively quickly (meters per hour or day). Water in aquifers typically moves much slower (sometimes just meters per year).
• Structure: subterranean rivers exist within open spaces like caves and tunnels. Aquifers consist of water-saturated rock and sediment without large open spaces.
• Formation: subterranean rivers form primarily through dissolution of rock. Aquifers form when water accumulates in porous materials like sand, gravel, or fractured rock.

Both are important sources of freshwater, but they behave differently and require different approaches for access and management. Learn more about the natural water storage systems that supply drinking water to billions in our other article.

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