Additionally, underground rivers find their way into caves through entrance holes. They make the cave bigger though erosion. When an underground river exits a cave, it is called a spring. Apart from the limestone caves, some caves are formed by a hydrogen gas. It comes from deep below the Earth and rises to the surface. The gas dissolves or eats away at the rock and creates a cave. Other caves are sea caves. They are found along coastlines. The waves pound the coastline, eroding weak areas in the rock.
Whether in limestone, in hard granite rock or along coastlines, most caves share similarities in how they are formed. They are most commonly eroded by acid, gas or water erosion. But in limestone caves, a unique feature is found: stalactites and stalagmites. These are limestone formations that grow in pairs. Stalactites are long pointy formations hanging from cave ceilings. Over time, the limestone acid drips onto the floor of the cave. The acidic water drips from the roof of the cave, making the stalactite thinner at one end. This creates a stalagmite-the exact opposite shape growing from the floor, not the ceiling. The tallest stalagmite can be found in a cave in Cuba. It is 67.2 meters tall.