During the last week, have you noticed a sewer-like smell in the Saddleback Valley? It turns out all of Mission Viejo has been stinky the last few days, and according to the OC Register, the reason has to do with... microscopic organisms. In this case, a species of algae, producing the very same gas that emanates from sewage: hydrogen sulfide. (Thus the "sewer gas" smell.)
The foul-smelling algae has been surfacing in the Upper Oso Reservoir. The Upper Oso Reservoir is a man-made lake near the intersection of Los Alisos Blvd. and the 241 tollroad. (The reservoir was constructed by building a dam across Oso Creek, a creek which starts in the foothills of the Santa Ana Mountains and flows down toward Laguna Niguel. It's the same creek that feeds into Lake Mission Viejo).
You can read more here, at the OC Register's website.