Step 1- Rain soaks in the soil and goes though the cracks.
Step 2 - When it rains it makes puddles and goes though the cracks and it dissolves and makes carbon dioxide bubbles.
Step 3 - When it rains water goes through the cracks and makes tunnels.
Step 4 - When it rains more it makes bigger tunnels.
Step 5 - When it rains more they make stalactites and stalagmites and more tunnels.