Sensor networks will shortly consist of globally deployed sensors providing real-time geo-centric information to users. Particularly, users with mobile devices will issue ad-hoc queries usually from within, or nearby, the queried area. In this paper, we propose an in-network Data-Centric Storage (DCS) scheme, namely the Spatio-Temporal Data-Centric Storage (STDCS) scheme, to efficiently answer these mobile user queries. STDCS is designed to maintain load-balancing among sensors to cope with query hotspots in the network. It is different from previous proposals in two aspects. First, it is based on the novel idea of using a temporally evolving spatial indexing scheme to balance querying load among sensors. Furthermore, STDCS uses dynamic mechanisms for query hotspot detection and decomposition. We conducted extensive simulations that showed our scheme's superiority to both local storage and spatial indexing (the only known geo-centric storage schemes) whenever experiencing query hotspots of different sizes.