Seine-St-Denis has organized the the 24 hours of biodiversity since 2010. Once a year, over a weekend or a day, the knowledge instrument provides an opportunity for the inhabitants of the area to discover biodiversity and participatory science in a fun way. The first to test this format in France was the Seine-Saint-Denis Departmental Council in the framework of its Departmental Observatory of Urban Biodiversity (Observatoire Départemental de la Biodiversité Urbaine, ODBU) in cooperation with the Regional Agency for Biodiversity (previously Natureparif), the National Museum for Nature History and municipalities in the Department. Since 2014, the National Collectif of Participatory Science - Biodiversity is also involved. The Council organizes nature outings, workshops to discover flora and fauna, scientific inventories, night-time observation sessions and activities on the harmonious management developed on the Natura 2000 sites in Seine-Saint-Denis. The programme for these 24 hours also includes a census of the species in the department by each interested citizen, and a meeting with artists from the department to show the effervescence of this initiative and the presence and importance of nature in the city. All the participants are then invited to a "25th hour" during which the results of the day's inventories are summarised. The artists and work produced during the 24 hours is also presented. The objective of this scientific mediation action is to bring together the general public, environmentalists, nature conservation NGOs, scientists and artists from the department around the issue of urban biodiversity and its challenges, particularly in the department of Seine-Saint-Denis, and at the same time to allow everyone to discover the wealth of the department's natural heritage. The citizen science initiative has since been replicated in other cities in France, such as Rennes, Missilac, Odonat, and Brière.