A tank (10) for storing a liquid with a device (11, 12) for feeding one or more substances thixotroping the liquid to the liquid in the tank (10) is characterised in that the device (11, 12) is designed so that it feeds the thixotroping substance or substances to the liquid in the tank (10) as a result of a signal from a signal transmitter, so that the liquid completely thixotropes, at least in a closed outer layer inside the tank (10) that cannot be penetrated by a residue of the liquid, not thixotroped if necessary. This ensures that by simple means the liquid present in the tank (10) is pretended from being discharged or from escaping under certain defined conditions, particularly in emergency situations.