Sulfur-bearing molecules are widely used astrophysical probes in star-forming regions tracing the physical properties (density, temperature, kinematics) and the chemistry of the gas. However, observations of sulfur-bearing molecules in dense cores find a total abundance that is only a small fraction (~0.1%) of the sulfur seen towards diffuse regions. Thus, unlike all other major atomic species (hydrogen, oxygen, carbon, nitrogen) to this day we still do not know what species is the major reservoir of sulfur in the dense ISM. This has significant implications on our ability to reliably use these molecules as probes. Recent observations using the IRS instrument on Spitzer have potentially discovered the missing sulfur in atomic form lighting up at 25.2 microns within non-dissociative shocks in close proximity to 2 of youngest protostars (Class 0 objects). We propose here to survey additional Class 0 objects to determine if this result is peculiar to these objects or whether we have indeed found the primary reservoir of sulfur in dense interstellar gas. We will also explore the implications of these results on chemical theory using state-of-the-art chemical models. These observations will be a Spitzer legacy to ISM science and will offer the opportunity in the future to use S I emission as a probe of dense gas physics when its higher lying transition can be accessed by future observatories such as SOFIA.< Leer menos