Water is an environmentally-friendly refrigerant but the vaporization phenomena occurring when under low pressure in evaporators are not well documented. This communication aims at unveiling experimentally some features of the vaporization of water drops on a heated horizontal surface at a controlled pressure. A heat flux sensor was used to record the thermal signature of the process that was observed with a high-speed camera. The liquid volume was controlled by a peristaltic pump and injected through a needle located at a variable height. The influence of the temperature of the surface was investigated. Four regimes of vaporisation were distinguished. Conditions for their occurrence were discussed. One remarkable regime ("explosive boiling") is characterized by the bursting of a large bubble generated inside the drop, which results in atomization of the mother drop and spreading of the daughter drops. Explosive boiling might have the potential to increase the efficiency of low-pressure evaporators.