What was that black smoke in season 3 that killed Mr. Echo?

I don't think it was the Black Smoke that directly killed Eko. The Black Smoke had something to do with it, but whatever makes that crane noise (maybe that's part of the black smoke) is what flung Eko up in the air and did him in. The "monster" is pretty crazy. It's like a AI that you can see your past in, but then it also has some mechanical thing after it. Remember when Eko saw flashes from his past in the black smoke? If the black smoke is so dangerous why didn't it kill Eko then, and why didn't it kill Locke? Yes....sooooo many unanswered questions. lol

I don't know! I think something connected to his past...

wish i could know too... i'm just as much confused as u. i want to know whether all of the visions seen by eko in the island are the result of the black smoke? if so then what about the visions that locke see's about boone? is it the result of the black smoke too???

Great question! I wish I knew the answer, though! I'm so afraid that they're not going to have enough viewers when they come back from this break, and that there are going to be sooo many unanswered questions if they cancel the show!!!

we don´t know yet.but when this season over,i think that probabily we know wich is...