I looked up in this online dictionary for the word, endeavour,
but the example sentence even made me more confused.
Here is the example sentence:
In spite of our best endeavours, it has proven impossible to contact her.
Who has proved that it is impossible to contact her? Either her family or some rescue team, isn't it? So, I am wondering why it is not
In spite of our best endeavours, it has been proven impossible to contact her.
Please help me. I am trying very hard to understand why the example sentence given by the online dictionary is grammatical.