Since our daily activities are dominantly indoor, as smartphones emerge as the most popular personal computing companions, major IT companies recently launched aggressive investment on mobile indoor location services and positioning systems, e.g., on iOS or Android mobile devices. However, one major hurdle has not been conquered yet: smartphone-based high-resolution indoor localization. In this project, we propose a practical and accurate solution that fills the long-lasting gap for the first time (foot-level vs. meter-level with GPS, WiFi, or RSS). Without additional hardware components/extensions on users’ smartphones, the proposed Guoguo algorithm and system leverages pervasive microphones and adopts acoustic signals for smartphone-based fine-grained indoor localization. Experimental results demonstrate that the Guoguo system achieves high-accurate localization of 6cm and significantly outperforms most existing smartphone-based indoor localization schemes. For the first time, we can pinpoint a smartphone user within one foot, ushering in the dawning of enormous mobile indoor location and navigation services and applications.