Water can exist underground in two forms: groundwater and subterranean rivers. Groundwater is the water present in soil and in the cracks of rocks. When a rock can hold a quantity of water that is big enough to be used commercially it is called an aquifer. The depth at which soil or fractures in rock become completely saturated with water is called the water table. Groundwater naturally occurs in springs, and can form oases or wetlands. The high specific heat capacity of water and the insulating effect of soil and rock mean that groundwater can be maintained at a relatively steady temperature of 10°C.
A subterranean river is a river that runs wholly or partly beneath the ground surface. Subterranean rivers may be entirely natural, flowing through cave systems or rivers that are travelling down sinkholes to continue underground. In some cases, they may emerge into daylight further downstream.
Puerto Princesa Underground River is one of the few subterranean rivers, it has a second floor, which means that there are small waterfalls inside the cave. The cave’s dome was measured at its highest of 300 metres above the underground river. Within this caves, several large bats and cavefish were found. However, the deeper areas of the underground river are almost impossible to explore due to oxygen deprivation.
Another example of a subterranean river is Paul’s Underground River Cave, which is more than 24 kilometres long and contains an 8.2 kilometres long underground section of the Cabayugan River. The river winds through the cave before flowing directly into the West Philippine Sea and is navigable by boat. The cave includes major formations of stalactites and stalagmites, and several large chambers, including the 360-meter-long Italian’s Chamber with approximate 2.5 million square metres volume. Nine species of bats, two species of swiftlets and whip spider are found in the cave.