Limestone is a common type of sedimentary rock with a high calcium carbonate composition. Due to its calcium content, limestone is a soft rock that erodes easily.
Like other sedimentary rocks, limestone forms from layers of other material that undergoes compression over time. Because of its calcium carbonate, limestone is an organic sedimentary rock, and much of the calcium in limestone comes from the remains of ancient marine life, such as coral. Limestone also often contains silica from the skeletal remains of sponges and diatoms.
Limestone's vulnerability to erosion means that underground limestone deposits easily wear away with the presence of groundwater, forming tunnels and caverns. Regions with a great deal of limestone often have karst topography: rough surfaces above ground and cave systems below ground. Limestone erosion is responsible for the formation of some of the world's most spectacular cavern systems and cave formations. As calcium-laden water drips from cavern ceilings, some calcium is left behind each droplet, while some drips to the floor with the water. Calcium deposits on cave ceilings eventually become stalactites, while calcium build-up on a cavern floor leads to stalagmites. The two structures sometimes join to form a column. If limestone experiences substantial pressure, it becomes the metamorphic rock marble.