In this paper, we introduce a mobile application (app) that helps people practicing their English pronunciation using mobile devices. Equipped with an embedded assessment engine, the app offers accurate pronunciation assessment and feedback to a learner instantly. Moreover, game elements and mechanics are introduced to make the training experience fun, rewarding and engaging. The app turns out be phenomenal and leads to massive pronunciation training at an unprecedented scale. Since its launch, the app has accumulated more than 20 million users. Hundreds of years of speech data are collected from more than 11 million different speakers, which is probably the largest speech corpus for Chinese spoken English in the world.