请分析下面这个句子的成分,主句是个强调句型,survive后面的我就搞得不清楚了.

请分析下面这个句子的成分,主句是个强调句型,survive后面的我就搞得不清楚了.
it is almost alway due to some very special circumstances that traces of land animals survive,as by falling into inaccecible caves,or into an ice crevasse,like the siberian mammoths ,when the whole animal is sometimes preserved,as in a refrigerator.