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