|Under the Ground|
Most of the Earth's land has now been surveyed and mapped but there are many caves under the ground which have still to be explored. And there may be more still to be discovered. The biggest cave systems of all are found in thick layers of a rock called limestone.
It is rainwater, seeping into the ground, that dissolves, or eats away, rocks such as rock salt. When rainwater contains carbon dioxide from the air and the soil, it acts as a weak acid on certain types of rock, such as limestone.
Animals which live in caves use their senses of smell and touch to find their way around in the dark.
In the Mammoth Cave National Park in Kentucky, USA, the cave system is about 560 km ( 348 miles ) long.
The Sarawak Chamber in Malaysia is the world's biggest Cavern. It is 700 m ( 2,297 ft ) long and over
70 m ( 230 ft ) high.
Rivers and Glaciers