Pope Benedict XVI issues his final Twitter message.

 Pope Benedict XVI, the first pope with his own Twitter account, tweeted a final goodbye Thursday morning before taking the almost unprecedented step of retiring from the papacy.

Benedict left the Vatican onboard a helicopter bound for a seaside papal residence near Rome at 11 a.m ET. Benedict's retirement will become official at 8 p.m. ET Thursday, when he will gain the newly created title of "pope emeritus."

Benedict promised in a Thursday speech "unconditional reverence and obedience" towards his yet-unchosen successor, who will be picked by a conclave of cardinals expected to begin deliberations in a few weeks. It has been 600 years since a pope retired from the position, making this particular conclave a unique event in modern Catholicism.