Many ancient shipwrecks have been discovered at the bottom of the sea.

shipwrecks 是名词 shipwreck 的复数形式。ship 意思是"船"，wreck 意思是"残骸、失事"。shipwreck 合起来指"沉船残骸"，即因事故或战争等原因沉没的船只及其遗留物。