what a relief it was when the boulders suddently disappearanced

what a relief it was when the boulders suddently disappearanced！

what a relief it was是主句，感叹句式。

when the boulders suddently disappearanced！
指代it，为主语从句。

当那块大石头消失的时候，那是多么让人觉得轻松啊！

what a relief it was+when the boulders suddently disappearanced

后面的when the boulders suddently disappearanced作为伴随时间状语，前面的what a relief it was是个what...it is形式的感叹句