Since 2010, the sporadic population booms of native, coral-eating crown-of-thorns starfish (Cots) have been affecting the survival of Australia's Great Barrier Reef - but help is at hand. RangerBot, an underwater drone unveiled in August, is touted as the latest weapon to hunt and kill the marauding sea stars. It is a collaboration between Queensland University of Technology (QUT), Google and the Great Barrier Reef Foundation. Infographics journalist Lim Yong dives in for a peek at this "silent assassin", the world's first underwater robotic system designed specifically for coral reef environments.