Todd Sidon, of Ardgowan School, asks :-
Why are there rivers underground? Chuck Landis, a geologist at Otago University, responded.
Underground rivers form in limestone rock. Underground water is commonly slightly acidic, and this makes it possible to dissolve limestone. No rock types other than limestone are easily dissolved like this. The rivers begin in tiny cracks in the rock. As the walls of the cracks dissolve, the cracks become wider and wider. They also may join with other cracks. This way, tiny underground streams become larger and might eventually become underground rivers. When the water level lowers, the upper parts of river channels become filled with air and caves are formed. Springs in limestone country are commonly formed at places where underground streams come to the Earth's surface.
It is common to hear people speak of underground rivers in areas where limestone is not present. These are probably not actual rivers, like those in limestone, but slow-flowing water moving between pebbles or sand grains in underground sediments or along tiny cracks in the rocks like sandstone, granite, etc. In these cases, there is no large open space, like a river or cave, just many interconnected tiny spaces through which the water slowly flows.
True underground RIVERS are quite uncommon and have not yet been found in New Zealand. However there are many underground streams known in New Zealand. These are best developed in northwest Nelson, northern Westland, and Waikato but they do occur elsewhere (for example near Te Anau, Oamaru, and Timaru).