Groundwater moves through porous rock formations, similar to water flowing through a sponge with interconnected pores. In nature, no space remains empty. Therefore, the pore space within the underground rock formations, no matter how small, remains filled either with air or water (sometimes oil and gas in deeper formations). Given a continuous supply, water enters a porous rock formation replacing the air and gradually saturates all the pore spaces. As the process continues, excess water moves through the saturated formation under gravitational force. Water can even seep through poorly cemented house walls and concrete basements, particularly during the rainy season.

Therefore, when a storage space such as a dug well is constructed within a saturated rock formation, a part of the water from within the formation flows out as free water (specific yield) and gradually accumulates to fill in the well till it reaches a level equal to the water level in the formation. Water flows through joints, fractures and contact zones between two hard rock formations. Sometimes, one can even see water flowing out through a fracture or a contact zone in a dug well constructed in hard rock formation tapping such zones giving credence to the erroneous notion that groundwater moves underground as a subterranean stream.

Carbonate rocks are also known to develop due to the action of water, large-sized cavities and inter-connected solution channels, which contain and transmit large quantities of groundwater. Sometimes, in coal mines, groundwater accumulated in large quantities in spaces created due to coal removal can cause accidents, thus creating the effect of a flowing underground river. Coal mines can also get flooded due to the direct water inflow from surface water bodies.