What is the fine-grained topic of the following text: Since then, the wreckage of B-52 in Huu Tiep Lake became the symbol of Dien Bien Phu victory in the air and the pride of the people of Ngoc Ha as well as the entire Vietnamese nation. Only return the topic.
Wreckage of B-52 in Huu Tiep Lake as a symbol of Dien Bien Phu victory