A gaming machine (100) may determine that a trigger event for a bonus game occurred during a base outcome event of a base game. The base game and the bonus game may both be reel-based games being executed on behalf of a client machine. The symbols (600; 700; 800; 900; 1000; 1100) on each reel may include at least two unstacked wild symbols. In response to determining that the trigger event occurred, an instance of the bonus game may be awarded. A special symbol (704; 902; 1102) may be included on each reel for the bonus game. Until all of the reels contain stacked wild symbols, iterations of bonus game operations may be repeatedly carried out. These operations may include determining a symbol set (600; 700; 800; 900; 1000; 1100) for display on the plurality of reels, where any special symbol appearing on a particular reel results in the wild symbols on the particular reel being stacked.