The Natural tunnel began to form more than a million years ago when groundwater bearing carbonic acid percolated through crevices and slowly dissolved limestone and dolomite bedrock .
The Natural tunnel is up to 200 feet ( 61 m ) wide and 80 feet ( 24 m ) high .
The Natural tunnel , which is up to 200 feet ( 61 m ) wide and 80 feet ( 24 m ) high , began to form more than a million years ago when groundwater bearing carbonic acid percolated through crevices and slowly dissolved limestone and dolomite bedrock .