stumble across/on/upon something/someone
phrasal verb with stumble verb uk/ˈstʌm.bəl/ us/ˈstʌm.bəl/
C2 偶然发现；撞见；邂逅
to discover something by chance, or to meet someone by chance
Workmen stumbled upon the mosaic while digging foundations for a new building. 工人们在挖一栋新楼的地基时偶然发现了那幅镶嵌画。