Other
Colorado Geological Survey: Gradient Map
This site provides geothermal maps of Colorado with the intent being to identify possible sources of geothermal energy. It explains how the temperature measurements were taken and the reasons the results must be considered to be...
University of Calgary
University of Calgary: Energy Education: Geothermal Gradient
A good, concise explanation of the geothermal gradient and how and why the Earth's temperature gets hotter and hotter with increasing depth. Includes two diagrams showing this and the heat transfer mechanisms that are taking place.
Indiana University
Indiana University Bloomington: Geo Notes: Geothermal Resources in Indiana [Pdf]
Describes how geothermal energy is harnessed in Indiana.