Formation of self-organized nanogratings in bulk glasses with femtosecond laser pulses is one of the most intriguing phenomena in the interactions of light with transparent materials. With the feature sizes far beyond the optical diffraction limit, these nanostructures have found widespread applications in nanophotonics and nanofluidics. The physics of the phenomenon is still far from being fully understood, largely because of the lack of a technique for noninvasively visualizing the formation of the nanogratings embedded within bulk glasses. Here, we access the snapshots of morphologies in the laser-affected regions in a porous glass that reveal the evolution of the formation of nanogratings with an increasing number of laser pulses. Combined with further theoretical analyses, our observation provides important clues that suggest that excitation of standing plasma waves at the interfaces between areas modified and unmodified by the femtosecond laser irradiation plays a crucial role in promoting the growth of periodic nanogratings. The proposed universal nanostructure growth mechanism involving laser-induced plasma wave formation at the interfaces of the seed structures may tie together many previous observations, meanwhile linking the in-volume nanograting formation to previously discovered mechanisms for surface nanoripple formation.