8. Amid rumors that there were prehistoric mammoths wandering around the unknown region and that somewhere in its wilds was a mountain of rock salt 80 by 45 miles in extent，the two captains set out．

(同位语从句rumors that…)

当时有一些传言，说有史前的猛犸在这一陌生的区域活动，而且在这一区域的某个地方有一座巨大的盐石山，其面积达80英里长50英里宽。就在这些传言中，两位上尉出发了。

分句1：Amid rumors

分句2：that there were prehistoric mammoths wandering around the unknown region

分句3：and that somewhere in its wilds was a mountain of rock salt 80 by 45 miles in extent

分句4：the two captains set out

分句1和分句4构成倒装，正常的语序应该是分句4+分句1，即the two captains set out Amid rumors。分句1嵌套分句2和分句3，分句2和分句3是修饰分句1的rumors的同位语从句，用来说明谣言的具体内容。分句2和分句3构成并列关系。