 Pritzker said the residents of Madison County deserve a thorough and well-communicated investigation. The city of Alton has closed the park through the weekend while an independent geologist, employees of New Frontier Materials and others investigate the entire park where mining has taken place. The large sinkhole opened up under the new turf fields Wednesday morning near the front of the park swallowing a light pole. The hole is about 100 feet wide and more than 50 feet deep. Mark Ellerbrock, Big Z Radio News.