Underground rivers

Rivers flow underground as well as on the surface. Whole rivers can disappear underground when a river runs over permeable rock. This is because permeable rocks such as limestone have small holes in them. The water drains through these holes until it reaches a rock which has no holes in it.

Caves are often formed in permeable rocks. The water from the river erodes the sides of the caves, making them even bigger. The river often runs through the middle of these caves.

Stalactites and stalagmites
Stalactites are sometimes found on the roofs of caves. They are formed from water dripping through the limestone. Stalagmites grow up from the floor of the cave, where water drips down from the roof. Many people explore caves with underground rivers. This is a dangerous hobby.

Interesting fact
The Echo River flows through the Mammoth Caves in Kentucky in the USA. The caves are 110 metres below ground.