The Undertavern

From the introduction: John Jentilman owns the Two Moon Tavern, a cheerful if simple establishment. Always an odd sort, he constantly complains about rats in the cellar of the tavern. After coaxing the party into helping, he ushers them through the iron cellar door... and locks it behind them. Trapped in a bare room with a well in the center of the floor, they must seek a way out of their predicament.