这句话错在 which underling.
Which 后面应该跟一个动词,所以应改为：which underlies.
还有一种改法,是直接去掉which,这样变成了：the vital element underling ...把从句变成一个动名词来修饰the vital element.
我的意思是：这句话有两种说法.
1.However,it is really hard to say that the increasing consumption of petrol is the vital element which underies all of the environment problems.
2.However,it is really hard to say that the increasing consumption of petrol is the vital element underling all of the environment problems.
清楚了吗?