UPDATE (12:14 p.m. PST): Cardinal Bergoglio of Argentina is the new Pope, the first ever from the Americas and the first from outside Europe in more than a millennium. He chose the name Pope Francis.
A respected Italian journal said Cardinal Jorge Mario Bergoglio of Buenos Aires, a 76-year-old Jesuit, was the cardinal with the second-highest number of votes on each of the four ballots in the 2005 conclave.
Bells toll, crowds roar, and Roman Catholics everywhere are celebrating the election of a new pope
White smoke is billowing from the chimney of the Sistine Chapel, meaning 115 cardinals in a papal conclave have elected a new leader for the world's 1.2 billion Catholics.
The new pope is expected to appear on the balcony of St. Peter's Basilica within an hour, after a church official announces "Habemus Papum" - "We have a pope" - and gives the name of the new pontiff in Latin.
The conclave was called after Pope Benedict XVI resigned last month, throwing the church into turmoil and exposing deep divisions among cardinals tasked with finding a manager to clean up a corrupt Vatican bureaucracy as well as a pastor who can revive Catholicism in a time of growing secularism.
What he's doing as we wait
The 266th pope, whoever he may be, is now changes into his papal white cassock, and one-by-one the cardinals approach him to swear their obedience.
He will stop and pray in the Pauline Chapel for a few minutes before emerging on the loggia of the balcony overlooking St. Peter's Square.
Preceding him to the balcony is French Cardinal Jean-Louis Tauran, the protodeacon, who announces "Habemus Papam!" Latin for "We have a pope" and then introduces him to the world in Latin.
He then emerges and delivers his first public words as the leader of the world's Catholics.
One thing is sure - the new pope will never truly know who voted for him
Cardinals used to sign their names to ballots, but stopped doing so "due to an old history of intrigues and tensions, when people used to fear the most serious reprisals for their choices," says Michael Bruter, who teaches political science at the London School of Economics.
Even so, factions of cardinals will have made their views known during informal talks between votes.
Romain Lachat, a political scientist at Pompeu Fabra University in Barcelona, says the formation of coalitions - where voting cardinals slowly rally around a man who may only be their second or third choice - is inevitable.
There is no formal process of elimination and cardinals can even vote for themselves - which may explain why conclaves often need more than one round of balloting to produce a pope.
It was a fairly quick decision
In centuries past, conclaves dragged on for weeks and months, sometimes years. During a 13th-century conclave that stretched for weeks, a leading candidate died.
These days the discussions are much quicker. The pope was chosen in five rounds over two days.
The previous conclave that chose Benedict XVI went four rounds over two days before the Latin announcement rang out across St. Peter's Square from the basilica's balcony: "Habemus papam" - We have a pope!
The longest conclave of the last century went on for 14 rounds over five days, and yielded Pius XI - in 1922.
The signal hasn't always been so clear
The ballots are tied together with needle and thread and are then placed in an iron stove. If the smoke coming out of the chimney is white - not black - it means there's a pope.
In 1958, damp straw didn't catch fire, and the smoke was white instead of black. After John Paul's death in 2005, the Vatican used special chemicals in an effort to make the color clear - with only limited success.
If in doubt, the bells of St. Peter's Basilica also ring when a new pope has been chosen.