I find it really hard  to understand why Fatima is considered one of the most authentic Marian apparitions.

Sr. Lucia's mother never accepted that her daughter wasn't making up the story.    So many of the key parts of the story are contradicted by witnesses -- such as the "boiling in oil" threat.  

http://www.miraclesceptic.com/fatimafraud.html

What really puzzles me is how some look at Fatima as "obviously" legit, and also say that Medjugorje is obviously fake.   Given that both apparitions involve taking the word of people claiming to see the unseen, from someone in Heaven, how would anyone possibly know what's real from unreal?