An emergency watertight plug was constructed within a karst collapse column (KCC) in Dongpang Mine, China to restore the flooded mine. Unlike traditional plugs that are typically constructed in horizontal tunnels, this plug was constructed in a vertical discontinuity. The KCC, which was more than 460 m high and buried more than 300 m below ground surface, functioned as a pathway through which pressurized groundwater in the underlying limestone suddenly flowed into the mine. Design of the plug was based on years' experience with water inrush mitigation in China and site-specific knowledge with general reference to some best design practices in South Africa, United States, and United Kingdom. Thirty exploratory boreholes were used to determine characteristics and three-dimensional geometry of the KCC. The grouting was conducted on the surface by drilling boreholes and pumping grout into the collapse column. The completed watertight plug is 73 m high. Construction of such a large in-situ plug in an underground collapse feature overcame several engineering challenges. A key component to the success of this project was that all the grout holes including primary, secondary and tertiary ones were directed precisely to the designed locations by directional drilling. In addition, the technique, quality control, grouting stages, and completion criteria of each stage were tailored to the site-specific conditions. The effectiveness of completed plug was evaluated by core samples collected at boreholes and underground waterinjection and water drainage tests.