Caves are underground holes in the rock. Some caves open up when the ground splits, as in an earthquake. Some are eroded by waves hurling stones and pebbles at a cliff. But most are made in limestone, a kind of sedimentary rock, by a chemical process. Rainwater naturally contains tiny amounts of acid. It trickles into cracks and reacts with the rock made of soluble calcium carbonate to dissolve it away, little by little. Over thousands of years, the small cracks are gradually widened into huge caves. The science of exploration and all aspects of caves is called speleology. 

Stalactites and stalagmites

Water seeping through the rocks dissolves the minerals that make up the limestone. If the water then drips into a cave or cavity it leaves a tiny amount of the dissolved minerals behind as it falls. Over time, the mineral deposits build up into limestone “icicles”, called stalactites, hanging from the cave roof. Stalagmites are mineral towers built up from water dripping on to the floor from the stalactites above. Sometimes they join the stalactites to form solid columns of rock, linking the ceiling of the cave to the floor.

Lava tubes

Some caves are formed not by water dissolving limestone but through volcanic activity. As lava from the erupting volcano flows downhill, its surface cools and solidifies, forming a hard crust. Inside, molten lava may continue to flow. If it drains away, a hollow tube remains. Lava tubes can be up to 15 metres (50 ft) wide—though most are narrower—and can be kilometres long. Examples of lava tube caves can be found in the Canary Islands, Australia and on Hawaii. Some have been discovered on the Moon and Mars.