The Macrocosms platform is the largest of the 3 platforms of the European Ecotron in Montpellier. It is made up of 12 identical and independent experimental units dedicated to research on ecosystems and biological communities. Each experimental unit consists of a transparent FEP dome (volume of ~40m³) in which atmospheric conditions are controlled. Designed to operate in natural light, the platform can also operate in controlled light conditions after covering the domes with an opaque film. Under each dome, a belowground chamber hosts a large instrumented lysimeter (2 to 12 tonnes, soil depth up to 2 m, canopy area 2, 4 or 5 m², canopy height up to 2 m), and the machinery for the environmental controls. The facility is unique for permitting accurate simulations of wide range of environmental scenarios as well as the measurement of evapotranspiration by weight loss and the fluxes of the major greenhouse gases (CO2, CH4 & N2O) at the ecosystem scale through changes in their concentration at the entrance and exit of a measured flux of air flowing through each dome. The gas measuring system remains flexible and offers the possibility to carry out additional measurements.