The expedition ECO2@NORTH SEA with the Irish research vessel CELTIC EXPLORER (20 July-6 August 2012) was conducted in the framework of the EUROFLEETS project that covered the ship time, costs for transport and travel. The first target of the researchers on board the CELTIC EXPLORER was the Norwegian gas field "Sleipner", which has been used for more than 10 years for CO2 in parallel to gas production. A specific feature of this cruise was that a CO2 release experiment was conducted in the vicinity of Sleipner at the seafloor to trace the spreading of a limited amount of CO2 in the sea water and to test the sensitivity of the sensors. In addition to Sleipner, the so-called blowout crater in the British sector of the North Sea was investigated, where high quantities of methane have been emitted for more than 20 years following a failed drill attempt. Ideally, this unplanned long-term release experiment can be used to estimate how much methane is emitted from the seafloor and its resultant distribution in the water column. Furthermore, it can be use as a natural laboratory to study the establishment of microbial communities, which are capable of consuming this methane and may hinder the emission of the green house gas into the atmosphere.