How does groundwater enlarge cracks in limestone into cave passages?

Groundwater enlarges cracks in limestone through a process of chemical dissolution, primarily driven by the action of carbonic acid.12

As rainwater infiltrates the soil, it absorbs carbon dioxide to form a dilute solution of carbonic acid. When this acidic water reaches the limestone bedrock, it reacts with the calcium carbonate (calcite) that composes the rock, dissolving it and carrying the material away in solution.34125

This process occurs in two main stages:

• Initiation Phase: Groundwater moves slowly through narrow joints and bedding-plane fractures. Because the water becomes nearly saturated with dissolved minerals, this initial enlargement is very slow, often taking thousands of years to widen a fracture to a few millimetres.6
• Conduit Evolution: Once a continuous pathway reaches a threshold width of about five to 10 millimetres, flow patterns shift from slow percolation to turbulent, pipe-like flow. Higher flow velocities bring more unsaturated water into contact with the bedrock, significantly accelerating the rate of dissolution and allowing the water to transport sediment, eventually forming large cave passages.62