Most New Testament scholars agree nowadays that Mark 16:9ff. is not the original ending of Mark. Either it ended with v.8, or there was an original ending that's been lost (sometimes thought to be something like Matthew's ending but with differences similar to how Mark normally is different from Matthew). A certain breed of skeptic often found on History Channel or Discovery Channel Easter specials will sometimes use this to claim that Mark doesn't actually report the resurrection, with the insinuation that Mark is the earliest gospel and therefore the most reliable reporting of events. Therefore, we might be expected to include, Christians invented the resurrection after Mark's gospel was fully composed.
Mark Heath nicely presents several reasons why such skeptics have to be ignoring what the Gospel of Mark really says and what else is in the New Testament. According to standard dating of Mark (by scholars across the theological spectrum), Paul's first letter to the Corinthians church is earlier than Mark, and chapter 15 of that letter is the lengthiest discussion of the resurrection in the entire New Testament. Furthermore, the entire gospel of Mark forecasts the resurrection and leads to its expectation, even explaining elements of it long before it gets to the actual events. But most importantly, the resurrection is the very last event reported in the section of Mark 16 that most scholars consider authentic. The disciples are told that he has been raised and told that they will see him. There aren't chronicles of what Jesus did after the resurrection, as there are in all three other gospels and in the book of Acts, but the resurrection is very clearly reported right there in the section that no one questions.
I'm less convinced on the fourth reason, so I'm not mentioning that here, but you can see Mark's post for it and my comment for my response.