When a river enters a karst landscape, it can disappear underground through grikes. The grikes become enlarged, forming big vertical holes known as swallow holes . Swallow holes vary in size depending on how large or small the river flowing into them is. Poll na gColm is an example of a swallow hole in the Burren.
Rivers can disappear underground through a swallow hole and re-emerge at some point further downstream.
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In pairs, look at this OS map of the Burren. Can you identify places on the map where swallow holes might exist? Are there any rivers that seem to disappear?