翻译：How some mammals came to live in the sea is not known.

一、翻译：一些哺乳动物是如何来到海中生活的仍不被大家所知。

二、语法：
1、该句为主系表结构
主语是由How引导名词性从句How some mamals came to live in the sea , 系动词是is , 表语是not known .

2、关于How引导名词性从句
how引导名词性从句有如下几种情况：
a. 表方式、方法，相当于the way in which。可译作“怎样、怎么”。例如：
I didnt know how he came here.

b. 表程度上的感叹，可译作“多么……、真……、这么……”等。例如：
He said how beautiful those flowers were.
He said how he regretted the hours wasted in the woods and fields.
He was surprised at how much water I had drunk.

c. 表对程度、数量、情况的疑问。例如：
She wondered how he loved her.
I dont know how many students there are in our school.
I have no idea how my mother is now.
I want to know how he is getting on with his studies.

d. 仅用作连词引导间接陈述句，本身无意义。例如：
He told me how (= that) he had read about it in the newspaper.

e. 用作连词，相当于the fact / situation / times... that, 可译作“……的样子”、“……的情形”