might never have been diacovered 是前面The cave 的定语。意思是这可能还没有被发现的洞穴
The cave(that/which) might never have been discovered 中省了THAT 或which.
The cave might never have been discovered 是主语 had not 谓语 entrance been spotted by the distinguished Frech pot-holer,Berger.宾语（ been spotted by the distinguished Frech pot-holer,Berger是entrance 定语）
这是新概念三上的文章，might 前时一个宾语从句省略that,真正的主语是the cave
虚拟语气的倒置句式。。。大意是说如果那个洞口没有被berger发现的话可能这个洞永远都不会被发现
你把might never have been discovered 去掉，这句子就好理解了
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