The book of Mark has two alternative endings, depending on the manuscript, indicating that one of those endings was not part of the original. The footnote on that scripture in the New International Version says:
"Mark 16:8 Some manuscripts have the following ending between verses 8 and 9, and one manuscript has it after verse 8 (omitting verses 9-20): Then they quickly reported all these instructions to those around Peter. After this, Jesus himself also sent out through them from east to west the sacred and imperishable proclamation of eternal salvation. Amen."
So that part of the scripture may not be 'authentic'.