Massively Multiplayer Online Science (mmos.ch) connects scientific research and video games as a seamless gaming experience. Players are invited to solve crowd-sourced research tasks integrated in video games to contribute to real-world scientific research: to understand the coronavirus by classifying cell populations, to analyse the DNA of the human microbiome or to search for new exoplanets. These mini-games have mobilised over 3 million citizen scientists from all around the world, who not just contributed with analysis, but created educational content and interacted with researchers. There are 3 billion gamers worldwide and the number is growing rapidly. By converting gaming time into useful activities in science, this citizen science project creates a virtually limitless resource for research, and a unique opportunity to engage these massive communities with science.