Revealing the dark biodiversity around us through a national school‐based citizen science project

School‐based citizen science holds great potential for expanding biodiversity knowledge while engaging young people in authentic scientific practices. This is particularly important for revealing ‘dark biodiversity’, defined as the vast yet often overlooked biodiversity that remains hidden due to its small size or cryptic nature, including microorganisms.

Here, we showcase findings from a nationwide citizen science project called ‘The Mass Experiment’, where 27,710 school students helped reveal Denmark's dark biodiversity, including bryophytes, lichens, and tardigrades hiding in plain sight. We provided teachers with professional support to facilitate inquiry‐based learning. Students in turn conducted standardized fieldwork in local ecosystems, collected samples and completed laboratory exercises using provided instructional materials and following their teachers' guidance. Identity of the student‐collected specimens was then verified by taxonomic experts at the Natural History Museum Denmark.

The project generated 4258 biodiversity digital records uploaded to the Danish biodiversity portal, including 69 bryophyte species, 29 lichen taxa (or 28 lichen species), and 55 tardigrade species, 41 of which were new to Denmark and at least nine to science. Students successfully identified most target bryophyte and lichen species using the project's guides.

Survey responses from more than 1000 students and 400 teachers revealed high levels of engagement, increased interest in natural sciences, and strong appreciation for contributing to real research, although some teachers found aspects of the protocol overly complex. The project also achieved substantial public visibility, with 87 media features and numerous outreach activities.

Overall, this study demonstrates that large‐scale, school‐based citizen science can simultaneously produce high‐quality biodiversity data and foster meaningful educational outcomes, revealing hidden biodiversity while promoting environmental literacy, enriching museum collections and raising public awareness. In this way, the project also supports the Sustainable Development Goals proposed by the United Nations by bridging education, science and society.