Nature · Earth Science
Hidden worlds carved beneath our feet
A cave is a natural underground space large enough for a person to enter. Most great cave systems form in karst — landscapes of soluble rock like limestone where slightly acidic water slowly dissolves passages, chambers and spectacular mineral formations over tens of thousands of years.
Rain absorbs carbon dioxide to become weak carbonic acid. Seeping along cracks in limestone, it dissolves the rock grain by grain, widening joints into passages and rooms. Where the water table drops, air-filled caves remain, and dripping mineral-rich water builds stalactites and stalagmites.
Acidic water sinks through soluble rock, hollowing chambers and passages; dripstone grows from the ceiling and floor over millennia.
Dissolved out of soluble rock
The classic cave: water dissolves soluble rock along cracks to open vast chambers, then decorates them with dripstone formations built drop by drop over ages.
Tunnels left behind by flowing lava
As a lava flow's surface hardens, molten rock keeps flowing inside. When it drains, a hollow tube remains — sometimes long enough to walk through for kilometres.
Hollowed out by pounding waves
Where waves hammer a cliff, they exploit cracks and softer rock, prying it loose to hollow out a cave at sea level. Some open upward into dramatic blowholes.
Blue chambers melted within ice
Meltwater and air carve tunnels through glacial ice, and the dense ice filters light to a vivid blue. Beautiful but hazardous, they shift and collapse with the seasons.