Sinkholes in Illinois
March 13, 2013 | Illinois Geological Survey
The Illinois Geological Survey has a publication titled: Karst Landscapes of Illinois. From the publication "Two conditions are necessary for karst landscapes (green areas on map): (1) Soluble rocks, generally limestone and dolostone, must lie at or near the surface of the ground. (2) The loose soil covering the soluble bedrock must be thinner than about 50 feet."
Niagara Falls Without Water
September 5, 2013 | NBC News
NBC News has a collection of photo of Niagara Falls from 1969 when the flow of water to the American Falls was diverted over the Horseshoe Falls or to the Robert Moses power plant.
Photo Gallery: Ruby Falls Cave
April 6, 2014 | Weather Channel
The Weather Channel has a photo gallery that offers 21 views of Ruby Falls Cave in Tennessee. Several of the photos feature Ruby Falls, the largest and deepest waterfall open to the public in the United States.