1. It is thought l billion people, that is half the world’s workers, earn their living by farming.

　　It is/was thought + that 从句是一固定句式。 
类似的结构还有:It is / was reported / said / hoped / announced / believed / suggested ... + that从句,该句式亦可转化为: People said / hoped /suggested...+that从句。

　　It is thought that the food in this supermarket is cheap.

　　It is known to all that the earth travels round the sun.

　　2. They became experienced at finding underground springs.

　　be/become experienced at sth / doing sth意为“对某事/做某事有经验”。其中的at也可以用in;形容词experienced(有经验的)也可用作定语。例如:

　　She is an experienced teacher.

　　He is quite experienced in this kind of work.