Depth plot of maximum vertical hydraulic fracture propagation from all known shale gas operations in the Marcellus Shale from 2001 to 2010, with the base of the nearest overlying freshwater aquifer (vertical blue lines at the top of the diagram) plotted for comparison. Depth plot of maximum vertical hydraulic fracture propagation from all known shale gas operations in the Marcellus Shale from 2001 to 2010, with the base of the nearest overlying freshwater aquifer (vertical blue lines at the top of the diagram) plotted for comparison. The depth at which hydraulic fracturing was stimulated (sorted by depth for ease of reference) is given by the yellow line, while the red spikes represent the vertical extent of fracture growth (which is accurately known from micro-seismic monitoring). It is obvious that none of the induced fractures come even close to connecting with the overlying freshwater aquifers. (From Mair et al. 2012, with permission of the Royal Society and Royal Academy of Engineering, adapted after Fisher & Warpinski 2012.)‏ Paul L. Younger Quarterly Journal of Engineering Geology and Hydrogeology 2014;47:7-27 © 2014 The Authors