From ancient times, people have settled down along the banks of rivers, since they provide water to drink, to irrigate their fields, and to use as waterways to go from one place to another.
But hidden away, below the earth, are rivers that we rarely see, until they surface near the sea, or when they merge with another river. Some have names and are talked of with a sense of mystery, but many are nameless streams that flow through the nooks and crannies of the earth.
In India, the River Saraswati is believed to flow underground and join the Ganga and the Yamuna at Allahabad. It was once an important tributary of the Indus, but now is lost in the sands of the Thar desert, in Rajasthan. Most devout Hindus would, however, claim that it continues to flow underground.
What, after all, are these underground rivers?
Underground rivers are really just flowing reservoirs of rainwater. This rainwater percolates down through the earth’s surface to collect at a level where solid rock prevents the water from going any deeper.
As more and more rainwater percolates through, it increases the pressure on this trapped water. A slight crack, a small opening, is all that the water needs to push through and flow. Voila! You then have an underground river.
The flow of water within rocks depends upon two properties: the rock’s porosity and permeability. Porosity is that part of a rock’s volume, which is made up of empty space or with vessels that allow liquids to flow through. Crystalline rocks have low porosity which is why water finds it difficult to go through. However, sedimentary rocks have high porosity, hence water cuts through easily.
Permeability basically describes a substance that allows liquids and gases to flow through. Therefore, if underground water is surrounded by porous and permeable rocks, the water will push its way through easily.
Underground water follows the steepest slope down. Limestone rocks are honeycombed with tunnels and cones. When rainwater – which contains a certain amount of acid – falls on them, an interesting process takes place. The water starts flowing through the tunnels and caves seeking a way out.
As the water moves along, it wears away more and more of the surrounding rock to produce long tunnels, which eventually turn into underground rivers. These rivers finally reach the sea by surfacing at certain places.
Underground rivers are also born when a powerful spring is unable to surface because of the solidity and compactness of the uppermost layer of rock. It simply redirects its energy and starts flowing underground, looking for an alternate way out.
The distance travelled underground by such a river varies. In some cases it is more and in some others less. The famous Rhine of France travels only a short distance underground. The Saraswati travels completely underground, which is why many people doubt its existence.
Some people have made it their life’s work to study such rivers. Speleologists – or scientists who study and explore caves – have tried to follow the course of some underground rivers but many passages are too narrow to get through. What they do then is to mix a dye in the water so that when it surfaces they can accurately identify where the underground river started, and where it finally surfaced.