Short answers, pulled from the story.
Limestone is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. It is formed primarily from the skeletal fragments of marine organisms such as coral, shells, and microscopic plankton. These biological materials accumulate over millions of years to create the rock.
The primary architects of limestone have been living organisms for the last 540 million years. Corals, shells, and microscopic plankton have secreted the minerals that build vast underwater cities which eventually sank to become the bedrock of continents. This biological accumulation has been the dominant force in creating the limestone that covers large regions of the Earth's surface today.
Karst regions are distinctive erosional landscapes found where rainwater dissolves limestone to create pot holes, caves, gorges, and limestone pavements. These areas often lack visible above-ground sources of water because surface water drains downward through joints in the rock. Examples include the island of Öland in Sweden, which features alvars, which are extremely level expanses of limestone with thin soil mantles.
Limestone is crushed to create aggregate, the solid base for many roads, and is processed into cement, an essential component of concrete. It serves as the primary ingredient in the production of lime, which is essential for treating soils, purifying water, and smelting copper. The stone is also used as a white pigment or filler in products such as toothpaste, paint, and paper.
Limestone is partially soluble, especially in acid, because it is composed of calcium carbonate which slowly dissolves when it comes into contact with slightly acidic rainwater. This process creates karst landscapes and cave systems as groundwater carries the dissolved calcium away in solution. The same solubility that creates caves also makes limestone vulnerable to acid rain, which has caused severe damage to many limestone statues and building surfaces.