One of the world's most famous tourist attractions will take centre stage as the process begins to elect a new head of the Catholic Church after the death of Pope Francis.
Cardinals will convene inside the Sistine Chapel in Vatican City to vote in a papal conclave.
One of them will leave as the new pope.
It is a tradition shrouded in secrecy, but this is what we know about how it works.
It starts with daily meetings
Now Pope Francis has died, the cardinals will meet every morning for general congregations to discuss issues facing the church.
The new pope will be elected from the group of cardinals who will come together inside the Sistine Chapel to vote.
This is called the conclave, and it must start between 15 and 20 days after the "sede vacante" – a Latin phrase that means "with the seat being vacant".
Catholic Church historian Miles Pattenden says the election of a new pope is a "decisive moment that will define how the church develops".
What happens during the conclave?
The cardinals will vote to select a new pope once on the first day, and twice on each following day if no decision is reached.
A two-thirds majority is needed for a new pope to be elected.
Only cardinals under the age of 80 are allowed to vote.
Australia has just one cardinal, 44-year-old Melbourne Bishop Mykola Bychock, who became the youngest member of the College of Cardinals in 2024.
He is Australia's highest-ranking Catholic and the first Australian cardinal since George Pell.
Three other voting cardinals represent Oceania — John Dew of New Zealand, John Ribat of Papua New Guinea and Soane Patita Paini Mafi of Tonga.
During the conclave, the cardinals will stay at the Domus Sanctae Marthae, a guest house in Vatican City, with no communication with the outside world.
What happens during the conclave has become more secretive over time.
Dr Pattenden says more is known about what happened in conclaves of the 16th century, when all the major European powers had agents inside the conclave, than what takes place now.
He says the secretive nature of the conclave is important, both ideologically and theologically.
"The idea is that God, through the Holy Spirit, guides the cardinals to choose the right man as pope to make God's choice," Dr Pattenden said.
"If everybody knows the nature of all the discussions … that rather undermines that as a play."
How do the cardinals cast their votes?
Each cardinal writes his choice on a paper inscribed with the words "Eligo in summen pontificem", which is Latin for "I elect as Supreme Pontiff".
They approach the altar one by one to deliver their ballots and say:
"I call as my witness, Christ the Lord who will be my judge, that my vote is given to the one who, before God, I think should be elected."
Three scrutineers open the ballots and read them aloud.
As the scrutineer reads out each name, he pierces each ballot with a needle, binds the ballots with thread and ties a knot.
The results are added up and written down.
If no two-thirds majority has been reached, the ballots are then put aside and burned in the chapel stove along with a chemical, sending black smoke up through the chapel's chimney, which is visible from the outside.
The black colour indicates no new pope has yet been chosen.
If no result has been reached in three days, the sessions are suspended for a day to allow for prayer and discussion.
In 2013 it took two days to elect Pope Francis.
Dr Pattenden says conversations between the cardinals have likely been going on for "several years" in preparation for a swift conclave.
"There's a lot of pressure, particularly brought by the modern media cycle, to resolve things quickly, so they can't sit there as they sometimes did in the 18th century for six months until they can work out what the political solution is," he says.
Why everyone is watching for white smoke
If a two-thirds majority has been reached, the master of liturgical ceremonies enters the Sistine Chapel.
The senior cardinal asks the cardinal who has been elected:
"Do you accept your canonical election as Supreme Pontiff?"
In theory, a cardinal can refuse the papacy. But in all likelihood, these intentions would have been made clear in the course of the discussions leading up to the vote.
So, assuming the chosen cardinal says "I accept", the senior cardinal then asks: "By what name do you wish to be called?"
This time, white smoke pours out of the Sistine Chapel chimney, and the bells of St Peter's Basilica toll.
Inside the chapel, the cardinal announces what his name as pope will be.
The new pope then changes into his papal white cassock.
One by one the cardinals approach him to swear their obedience.
The new pope then stops to pray in Cappella Paolina — the Pauline Chapel.
Newly elected pope greets the world
The proto deacon, who is the longest-serving cardinal deacon in the College of Cardinals, then steps onto the balcony overlooking St Peter's Square and announces "Habemus Papam!" — Latin for "We have a pope."
The new pope emerges and delivers his first public words as pontiff to the waiting crowd.
Dr Pattenden says making a good first impression will be important.
"If you think back to 2013 when Francis came out there all smiles and waving, that was very important for kind of establishing that positive image of him which has been maintained over the last 10 years as a kind of humble, simple but good man," he says.
Beyond what the new pope has to say, Dr Pattenden says Vatican observers will also be looking at what language the new pope chooses to speak in.
He says it could serve as a powerful statement about "how he sees his place in the world".
What influence has Francis had on electing his successor?
Pope Francis has appointed about two-thirds of the cardinals who will vote in the coming conclave, including 18 new eligible cardinals in October 2023.
However, Dr Pattenden says cardinals are "free agents" and are not obliged to follow the previous pope's wishes.
"Even though the pope has an exclusive right to appoint the cardinals who then choose his successor, no pope has ever really been able to control the successor's conclave," he says.
In saying this, Dr Pattenden says it would be surprising if the next pope didn't, broadly speaking, share Pope Francis's views on social justice and the role of the church in the world.
