A sinkhole appeared south of Knoxville on 135th Place on the property of Elly and Ben Shaw Thursday. The sinkhole is in a pasture, and is approximately 50 feet wide and 25 feet deep. Marion County Engineer Tyler Christian has closed the road at that location until more is learned about the phenomenon. He strongly encourages people to avoid the area. After examining a KNIA/KRLS photo of the sinkhole, Jason Vogelgesang, a Geophysicist with the Iowa Geological Survey at the University of Iowa, tells KNIA/KRLS News that the sinkhole may be either a natural phenomenon, or related to coal mining activity. Vogelgesang says the Samuel Buckman coal mine that was in operation from 1885-1889 is located in the area, and that the sinkhole might be related to that mine. He adds that given that the S&S quarry is a limestone quarry, and that sinkholes sometimes appear related to limestone formations, it could also be a natural phenomenon. If natural, it may be the largest natural sinkhole ever observed in Iowa. A geological team from the University of Iowa plans to visit the location next week. KNIA/KRLS News will report the results of the investigation.