Around 55 000 tons of chemical warfare agents (CWA) were dumped into the Baltic Sea after the World Word II. Official dumpsites are Bornholm Deep and Gotland Deep, but there is evidence for their presence also in other places on the seafloor. Their metal shells are corroding and leaking to the environment at a rate that has not been measured so far. Moreover, a large-scale leakage in case of disturbance would pose a serious biohazard to the entire Baltic ecosystem. Unaware of this risk, hydrotechnical installations and other sea-bottom activities such as trawler fishing are increasingly claiming space within the contaminated areas.CHEMSEA is a cooperation of leading scientific institutes and maritime decision makers,established in order to assess and minimise risks related to sea-bottom activities near CWA dumping sites.CHEMSEA will increase safety of underwater activities,by defining guidelines and educating the relevant stakeholders and administrations.For the first time CHEMSEA project will compile all available information concerning CWAs in the Baltic Sea – their known locations,environmental effects and the so-far practiced methods for both their detection and analysis.New methods for environmental effects analysis of CWAs will be developed and tested in laboratory conditions.CHEMSEA will fill not only methodological,but also physical gaps in CWA research,by screening for dumped munitions in the so-far not examined dumping site in the Gotland Deep and suspected locations, such as the Gdansk Deep. With help of intelligent underwater vehicles samples will be taken to identify the level of CWA degradation.The produced GIS maps showing actual munitions’ positions and the range of polluted sediments will be used to update nautical maps and will significantly contribute to maritime security,including fishery and exploitation of sea bottom.Ecological risk assessment,asking into account different dispersion scenarios will help maritime administration to assess the environmental effects as well as human health hazard in selected offshore areas in case of CWA accident, and handle responsibly in case of a major disaster.At the moment safety recommendations regarding CWA are regulated by number of national legislations,as well as HELCOM recommendations.CHEMSEA will propose updated and unified guidelines,including recommendations for CWA hot-spots where special procedures should be applied.On this basis, trainings will be organised for employees of the relevant authorities,environmental and fishery organisations and other key maritime stakeholders.Moreover,CHEMSEA partners will form a Baltic CWA advisory body for maritime administrations, acting at the same time as hands-on transnational analytical centre of CWA related hazards.Its task will be also lobbying for adoption of CHEMSEA recommendations into national legislation Achievements: CHEMSEA project has been acknowledged by its stakeholders as an extremely successful one. Our partners were not only taking a very active part in the international scientific scene dealing with the CWA problematic, but pushed forward the political debate which raised a broad medial response. The OPCW, IDUM, HELCOM MUNI, EU BSRS and almost all responsible national authorities closely collaborated with the project within these 3 years, working out new approaches and solutions. The project reviewed all available methods for analysis of CWAs and their degradation products from environmental and biological samples, and selected those most suitable, which were then introduced in participating laboratories, and their performance was tested internally. Afterwards intercalibration study has been performed to test the precision and efficiency of introduced methods. All participating labs successfully adopted unified methods. A sound database of distribution of CWA objects in theGotland Deep and Gdansk Deep and describing their environmental characteristics has been compiled both as a result of desk-top study and cruise data. At the known positions where chemical weapons were detected sediment and water samples were collected for chemical and biological analysis. Using remotely operated vehicles (ROVs) samples were collected at immediate vicinity of suspicious objects. All collected sediments were classified according to the distance from confirmed munitions, and distribution plan was constructed. Samples were analyzed by participating laboratories.Positions of the detected objects and descriptive data has been post-processed to produce comprehensive GIS maps.National procedures concerning reactions against threat posed by chemical warfare agents have been assessed. A BSR contingency plan was developed and consulted with the relevant authorities during the on-the-spot trainings and information events.Awareness-raising meetings for fishermen have been organised in Poland, Lithuania, Finland and Germany. Guidebook for Fishing Boat Crews, including easy-to-read flyer and stickers with pictograms have been prepared and distributed in Poland and Lithuania. Several recommendations such as Guidelines for safe disposal of sediments, Guidelines for operation in CWA contaminated areas were written, consulted with stakeholders and summarized in the “CHEMSEA Findings” publication. All the project results were immediately published on the highly informative www.chemsea.eu and www.chemsea.pl websites. Finally, CHEMSEA produced also a policy paper to better transport its recommendations to HELCOM and national ministries.Agreement on the foundation of the International Center of Excellence for Dumped Chemical Munitions (CoE-DCM) was developed and signed by all PPs, thus committing themselves to long-term cooperation within the results and investments obtained during CHEMSEA.