A large second-order nonlinearity [chi((2)) 1 pm/V 0.2 chi((2)) (22) for LiNbO(3)] is induced in the near-surface ( 4 microm) region of commercial fused-silica optical flats by a temperature (250-325 degrees C) and electric-field (E ~ 5 x 10(4) V/cm) poling process. Once formed, the nonlinearity, which is roughly 10(3)-10(4) times larger than that found in fiber second-harmonic experiments, is extremely stable at room temperature and laboratory ambient. The nonlinearity can be cycled by repeated depoling (temperature only) and repoling (temperature and electric field) processes without history effects. Possible mechanisms, including nonlinear moieties and electric-field-induced second-order nonlinearities, are discussed.