When rain passes through the atmosphere, it takes in or mixes with carbon dioxide, turning it into a weak carbonic acid. When the rain falls in limestone, it dissolves the calcium carbonate in the rock, turning it into calcium bicarbonate and taking it away. Holes and large cracks can form in the rock. Such a place is called karst. If a stream or river passes through it, it could be swallowed up by a hole called a swallow hole. It can go through a series of underground caves and may appear on the surface again. An example of a karst area is the Burren National Park, in County Clare, Ireland.