The first observations of dense HydroCarbon Clouds (HCCs) in JET divertor are reported. They confirm the existence of plasma grown nanoparticles in JET plasmas. Due to their low temperature, high density, and high recombination rate, high density plasmas, detached configurations and MARFEs present favorable conditions for the formation of hydrocarbon molecules and hydrocarbon nanoparticles. These HCCs, identified with the new wide angle InfraRed (IR) camera, are always present during strong deuterium fuelling. The implications of these hydrocarbon nanoparticles for tritium inventory are estimated which can be very significant for ITER. The HCCs are also clearly visible in the high density phases leading to the formation of MARFE instabilities and they appear at least a second before the density limit disruptions caused by these instabilities. Therefore, the observation of HCCs in the divertor region can be a very useful indicator to predict density limit disruptions and can therefore contribute to alleviate a major issue for the operation of ITER.