The doctrine's most frequent application is to bodies of water. Throughout the United States, most lakes and streams are maintained under the public trust doctrine, typically for the purposes of drinking and recreational activities.
Generally, the public trust doctrine also prevents individual property from extending to the ocean.
For more on the public trust doctrine, see this UC Davis Law Review article, this University of Arizona Institute of Environment article, and this California State Lands Commission document.
Some have also written about the potential conflict that fracking may have with the public trust doctrine. For some of those viewpoints, see this University of Texas Law Review article, this University of Puget Sound Law Review article, and this American Bar Association article.