This tensile specimen of a high strength, quenched and tempered steel was exposed to an aqueous environment containing hydrogen sulfide. The sulfide ion promoted adsorption of hydrogen produced by the corrosion reaction on the metal surface. The crack was initiated subsurface surface in the circular region indicated in the figure. The final fracture was a result of overload when the remaining ligament was insufficient to sustain the applied load. However, in highly susceptible materials, cracking can be surface initiated with multiple initiation sites.