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1、 Cave exploration, or potholing,as it has come to be known, is a relativelynew sport.
洞穴勘查——或洞穴勘探——是一项比较新的体育活动。
划线是修饰前面的or potholing的。

2、 For him, caves have the samepeculiar fascination which high mountains have for the climber.
对洞穴探险者来说,洞穴有一种特殊的魅力,就像高山对登山者有特殊魅力一样。
划线处类比说明。

3、 The cave might never have beendiscovered had not the entrance been spotted by the distinguished Frenchpotholer, Berger.
若不是法国著名洞穴探险家伯杰由于偶然的机会发现了这个洞口的话,这个洞也许不会为人所知。
划线部分的语法:省略if的条件句,表达虚拟语气。正常语序
If the entrancehad not been spotted by the distinguished French phtholer, Berger, the cavemight never have been discovered.

4、 They had to edge their way along this, sometimes wading acrossshallow streams, or swimming across deep pools.
   他们不得不侧着身子往前走,有时过浅溪,有时游过深潭。