新三49课有个语法问题，学霸们看一下

RT，这课第二段的结尾处的这句话 After her departure, Aunt Harriet discovered that there were piles of empty wine bottles of all shapes and sizes neatly stacked in what had once been Bessie's wardrobe. 中的 in what 是什么从句？或者是不是连接代词，帮我看看，谢谢！