接下来课文...it is very rarely that one comes across a dead body, except, of course, on the roads. (省略句)
it is very rarely…(特殊句型，强调状语)
come across=run into
接下来课文...They decompose and are quickly destroyed by the weather or eaten by some other creature...
一带other，一般用单数词， any(some)other: 任何一个
接下来课文...It is almost always due to some very special circumstances that traces of land animals survive, as by falling into inaccessible caves, or into an ice crevasse, like the Siberian mammoths, when the whole animal is sometimes preserved, as in a refrigerator. ...
traces=remains
as…as…
survive by
when...: 时间状语 （是 falling into inaccessible caves, or into an ice crevasse的时间状语从句）
refrigerator:冰箱 frefrige
接下来课文...This is what happened to the famous Beresovka mammoth which was found preserved and in good condition. ...