The Citizen Rescuers for Collections (CRESCO) project brings together a team of Belgian biology researchers and curators from the Royal Museum for Central Africa (Tervuren) and the Institute of Natural Sciences (Brussels) with twenty-six citizen scientists for the first time. They join forces to make hidden biological museum collections accessible by taking standardized photos of specimens and by transcribing hundred-year-old handwritten collection labels that cannot be processed using Optical Character Recognition programs. CRESCO has pioneered hybrid, online and on-site, citizen science approaches to public participation in digitizing biological museum collections. The project has strengthened the citizen science community of the two museums by achieving gender equality and by reaching out to Ukrainian refugees. It has delivered quality scientific data that could not have been obtained otherwise. This study analyzes the scientific, social, environmental, economic, and political impact of the CRESCO project and argues that on-site participation has significantly more social impact than online participation. It concludes that, in citizen science, museum staff must reflect on participation with ethical concern and attention, emphasizing the importance of physical presence and dialogue.