This letter proposes a novel mechanism for detecting and tracking large-scale phenomena such as spreading of wild fire and diffusion of bio-chemical material. The mechanism takes into account a two-tier grid structure in order to achieve flexibility and reliability. For flexibility, the mechanism initially constructs a proactive coarse-grained grid structure on the whole sensor field. Then, once a phenomenon happens, fine-grained grid structures are reactively established within cells of the coarse-grained grid structure where the phenomenon can be detected. In addition, with respect to reliability, small-scale cells of the fine-grained grid structures can provide the detailed shape of boundary of the phenomenon; to quickly deal with diffusion of the phenomenon the mechanism prepares fine-grained grid structures into next coarse-grained grid cells toward its diffusion direction.