Warning Spoilers!

What caused the Second Impact? What will cause the Third?

Fact

It was first thought that the second impact was caused by a meteor hitting the polar caps and melting the ice bergs partially submerging the world underwater. We find out that that was false.
Then we find out that the second impact was caused by an angel coming in contact with Adam...that was false.
So what was the cause of the second impact? Mankind's first contact with an Angel (Adam). They shrank Adam (white giant) into the small little embryo to reduce any further damage that would come from the residual effects of the Impact.

So what will cause the third impact? This one is confusing...see, Seele wants to provoke the Third Impact to carry on with HIP, so they send the angels to Nerv HQ to do??? See that's the tricky part, We know that contact with Adam won't do it...I mean Adam is Angel and Eva. Eva's fight Angels and nothing happens...they come into contact, but what the heck? There is no Impact. It doesn't make sense.

Theory
The weird thing is, all the Angels head for the Central Dogma. That is where Lilith is. Now the Angels are supposed to go to Adam, but they are lead to Lilith...why? Could it be that Lilith is the cause of the impact from contact and not Adam...the whole time we have no clue to where Adam is. In the end when Kaoru comes up to Lilith he stops and realizes hey it's Lilith, And all of a sudden he has a change of heart, and asks Shinji to kill him. The whole time Kaoru never touches Lilith...so maybe it is Lilith that causes the Impact upon contact.

FACT and theory:
Update Third Impact is caused by the Angels coming into contact with LILITH! Not Adam. If you notice nothing touches Lilith, not even Kaoru. He doesn't because he realized that it was Lilith and not Adam. What does that mean? Ok. All things that are made from Adam must return to Adam. That's what Kaoru said. Well Adam is the first Angel and according to Gendou the first human. Ok, Kaoru is an Angel. So he must be something made from Adam or something like Adam. So he's off searching for him, but is actually lead to Lilith. This also confused Kaoru. Then he realized. "OH! It's Lilith. I see." Those that were made from Adam were made deliberately...i.e Eva. Those that came from Lilith are made from free will. People came from Lilith, which is why KAoru called us Lilims. Lilith is also known as Mother Nature. Kaoru (Tabris) who is the patron angel of "free will" lets humans decide for themselves their fate. Which is why he asked Shinji to kill him.

The reason why I say that Rei is also the 17th Angel is because she carries out what Kaoru didn't. She was the one who joined with Lilith.

Impacts in a nutshell

1st Impact - the destruction of the dinosaurs.
2nd Impact - Human's first contact with an Angel (Adam). They shrank him to an egg to reduce further damage from the initial contact.
3rd Impact - Supposedly the contact of an Angel with Lilith, but carried off by Rei and Lilith and Unit-01.

Source: Movie and TV series

Here's a quote from Misato:

Misato : They'll (Seele) set up the 3rd impact by using the Eva series and not the angels. The 2nd impact which happened 15 years ago was actually started by man. Reduction of Adam to the egg came before the other angels sensed what was happening in order to decrease damage to a minimum.