Current knowledge about the formation of calcite rafts was summarized and expanded with new data from the environment of a technical adit excavated in the 1950s in the central part of the Moravian Karst (Czech Republic). The location is within the temperate zone with an average annual temperature of 7–8 °C and an annual precipitation of 550–650 mm. Two different types of calcite rafts were identified: fine floating rafts (FF rafts) and massive calcite crusts (MC rafts). The transition from the FF raft to the MC raft requires the raft to be anchored to the side bank of the water pool. Once the FF rafts are anchored, they can grow on both sides (top and bottom) of the original air-water interface to form MC rafts. A comparison of water evaporation with CO2 degassing under given conditions showed that just degassing has been the key factor in achieving adequate supersaturation of drip water for nucleation of calcite at the air-water interface and for subsequent crystal growth. The conceptual two-layer model was designed and discussed in terms of the partial pressures of CO2 in the air and water in the adit. The discussion of the role of rafts in speleogenesis and paleohydrological reconstruction is supported by examples from the Sloup-Šošůvka caves (Moravian Karst). Display omitted •Calcite rafts are formed in an adit in the central part of Moravian Karst.•The microclimate in the adit corresponds to the microclimate of a static cave.•Two-layer model was derived to discuss the processes at water-air interface.•Degassing of CO2 was found to control calcite raft growth.•Water evaporation was insignificant under given conditions.