Background Offshore cavernous boulder reefs in shallow waters have a high biodiversity and are a rare and biologically important subtype of reef habitats. The reefs are listed in the Habitats Directive. In Denmark, however, cavernous boulder reefs in shallow waters have been extensively exploited; they have been targeted for their high concentration of easy-to-excavate large boulders that are suitable for constructing sea defences and harbour jetties. It is estimated that at least 34 km2 of boulders from predominantly shallow cavernous reefs have been excavated from Danish waters (i.e close to 100% of the area of this habitat). The national monitoring programme indicates that only five hectares are left untouched. One Site of Community Importance has been selected at Kattegat bay to be a sanctuary for donor populations and to provide a corridor linking sites within the Natura 2000 network. The restored site will make a significant contribution to maintaining reef-dependent populations in Denmark. Objectives The main objective of the project was to restore and maintain a favourable conservation status of the reef offshore habitat at Læsø Trindel and Tønneberg Banke in the Kattegat bay. It planned to achieve this aim by restoring the structure and function of the cavernous element of the shallow offshore boulder reefs and by stabilising the top of the existing boulder reef. As the project is a pilot project it would include a longer period of monitoring of the restored boulder reef. The project also planned to increase awareness among environmental managers, policymakers and the broader public on marine nature restoration, conservation and management issues. Results The project achieved its main objectives. It restored the target area using natural stones from a quarry in the southern part of Norway. The stones were distributed in the project site over an area of 5 ha. The restoration is estimated to have resulted in 6 tonnes of macroalgal vegetation and 3 tonnes of bottom fauna, as well as a surplus of nearly 700 million individual fauna. Changes in the fish community structure were also evident. Cod increased on average by three to six fold in the reef area, especially in the shallow part of the rebuilt cavernous reefs. The ecological benefit is expected to increase further in the years to come. The results of the restoration project prove that restoration of stone reef can be a valuable tool in obtaining and maintaining favourable conservation status in marine Natura 2000 sites. The increased biomass indicates that restoration of cavernous boulder reefs may be a feasible tool for implementing the Marine Strategy Framework Directive. The project, moreover, produced ‘codes of conduct’ that can inspire other areas in northern Europe to restore natural stone reefs. They contain valuable experiences and recommendations for carrying out restoration projects. For Danish initiatives, it also gives guidance on obtaining the necessary permission from different marine authorities. Many of the recommendations are also relevant for the restoration of other marine nature types, such as biogenic reefs. Following the monitoring programme, the prohibition of fishing at or around the restored reef was lifted. Bottom trawling, however, is a threat to stone reef and in order to protect the reef habitat the Danish AgriFish Agency is planning a prohibition for bottom trawling at stone reefs in Natura 2000 sites, including the Natura 2000 sites, Læsø Trindel and Tønneberg Banke. The goal is to stop trawling at the reef and in a buffer zone of 240 m. Finally, the project raised awareness among environmental managers, policymakers and the broader public on marine nature restoration, conservation and management issues. It organised an international symposium and produced a range leaflets, posters, signboards and articles about the project. The dissemination of marine issues and marine nature restoration is very challenging and much more difficult than land restoration. The production of a documentary video proved to be a very valuable way to disseminate marine nature restoration. Further information on the project can be found in the project's layman report and After-LIFE Conservation Plan (see "Read more" section).