Explosive desorption of guest molecules embedded in amorphous solid water upon its crystallization is known as the "molecular volcano." Here, we describe an abrupt ejection of NH_{3} guest molecules from various molecular host films toward a Ru(0001) substrate upon heating, utilizing both temperature programmed contact potential difference and temperature programmed desorption measurements. NH_{3} molecules abruptly migrate toward the substrate due to either crystallization or desorption of the host molecules, following an "inverse volcano" process considered a highly probable phenomenon for dipolar guest molecules that strongly interact with the substrate.