The Graves Were Opened

Jesus, when he had cried again with a loud voice, yielded up the ghost. And, behold, the veil of the temple was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent; And the graves were opened; and many bodies of the saints which slept arose, And came out of the graves after his resurrection, and went into the holy city, and appeared unto many. Now when the centurion, and they that were with him, watching Jesus, saw the earthquake, and those things that were done, they feared greatly, saying, Truly this was the Son of God.
- Matthew 27:50-54 (KJV)

It remains a complete mystery as to who these saints were, and what happened to them.

Some have suggested that they were the prophets, and went to heaven with Jesus.

No one knows for sure.