One of the pope's titles, Pontifex Maximus, means "supreme bridge builder". This title was formerly held by the pagan high priests of Rome and the Roman emperors, until the Christian emperor Gratian renounced it. (source)

It was not until the third century that it was asserted that Saint Peter was the first bishop of Rome. (source)

The son of Pope Hormisdas (pope from 514 to 523), Silverius, was himself elected pope in 536. Silverius was deposed nine months later and exiled to Palmarola, where he died three months later. Both popes were eventually canonized. (source)

Pope Stephen II was pope for only three days, from March 22^nd or 23^rd to March 25^th or 26^th, 752, dying of apoplexy before he could be consecrated. (source)

Pope Adrian II (also known as Hadrian II), pope from 867 to 872, was the last married pope. He had married before he was elected pope, and refused to put away his wife Stephania when he became pope. For a while he, his wife, and a daughter lived in the Lateran Palace together. Interestingly enough, several subsequent popes, though unmarried, fathered children. (source)

At the instigation of Holy Roman co-emperor Lambert, a "cadaveric synod" convened in 897, eight months after the death of Pope Formosus, to declare his five-year pontificate illegal and his acts null and void—chiefly the one establishing Lambert's rival Arnaulf as co-emperor. Formosus was exhumed and propped in a witness chair while the new pope, Stephen VI, served as prosecutor and a deacon represented the dead defendant. Found guilty, the corpse was stripped of papal array and tossed into the Tiber River. However, the Roman citizens, finding the trial somewhat unusual, had Stephen deposed and imprisoned. Pope John IX then nullified Formosus' conviction and had his body fished out of the Tiber and returned to St. Peter's. (source)

Perhaps the worst pope in history was Octavian, Count of Tusculum, who was consecrated Pope John XII on December 16^th, 955. On November 6^th, 963, Holy Roman Emperor Otto I summoned a council, levelling charges that John had ordained a deacon in a stable, consecrated a 10-year-old boy as bishop of Todi, converted the Lateran Palace into a brothel, raped female pilgrims in St. Peter's, stolen church offerings, drank toasts to the devil, and invoked the aid of Jove, Venus, and other pagan gods when playing dice. He was deposed, but returned as pope when Otto left Rome, maiming and mutilating all who had opposed him. On May 11^th, 964, he was apparently beaten by the husband of a woman with whom he was committing adultery, dying three days later without receiving confession or the sacraments. (source)

The youngest pope ever was likely Pope John XII, who was consecrated in 955 at the age of 18 or 19. By some accounts, Pope Benedict IX was only around 12 years old when he was made pope in 1032; however, while Benedict was young at his consecration, it was more likely that he was in his twenties. (source)

Gerbert of Aurillac, who became Pope Sylvester II in the year 999, was the greatest Latin scholar around the turn of the first millennium. In his youth he went to Muslim Spain to study philosophy and mathematics. His education made him so intellectually superior to the rest of his Christian contemporaries that for many centuries he was regarded as possessing mysterious powers of black magic and sorcery.

The first female saint formally canonised by the Vatican (as opposed to the older, "pre-congregation" saints that were not formally canonised) was Saint Wilborada, canonised in 1047 by Pope Clement II. She was an anchoress who warned the monks of St. Gall of an impending Hungarian invasion. However, being an anchoress, she was walled into a small cell and could not escape, and so was martyred by the Hungarians. (source)

There were two Thursdays one week in 1147. Pope Eugenius III travelled to Paris, and was scheduled to arrive on a Friday. In order that the Parisians could hold a celebration on Friday, a day of fast, Eugenius decreed that that day would be a Thursday. (source)

A pope gave Ireland to the King of England. Pope Alexander III, wanting to eradicate Celtic Christianity in Ireland, declared Henry II of England to be the rightful Irish sovereign. This papal declaration, issued in 1172, led to the English conquest of Ireland, which took several centuries to complete, by which time England no longer followed the Pope. Not until the 20th century did the Irish regained their freedom. (source)

Pope Celestine III was the oldest pope at the time of his election, being around 85 years of age when he became pope in 1191. (source)

Pope Innocent IV, pope between 1243 and 1254, first decreed that cardinals should wear red hats at ceremonies and processions, in token of their being ready to spill their blood for the Saviour.

Prior to 2013, the only pope to abdicate of his own free will was Pope Celestine V. After Pope Nicholas IV died in 1292, the College of Cardinals were unable to agree on a pope for over two years, when they received a letter from Peter of Morone, a 79-year-old hermit, who wrote that God had told him that the cardinals would be punished for any further delay. The cardinals then hit upon the idea of choosing Peter, who took the name Celestine V when he became pope on July 5, 1294. He was not particularly well-suited for the position, and on December 13 he abdicated, citing "my lowliness, my desire for a more perfect life, my great age and infirmities, my inexperience, and ignorance of the world's affairs." His successor, Boniface VIII, annulled almost all of Celestine's edicts, except the one confirming the right of the pope to abdicate, and imprisoned him in Fumone Castle, where he died shortly afterwards. (source)

Due to a clerical error, there was no Pope John XX. Pope John XIX was pope from 1024 to 1032, and the next John to be pope was Pope John XXI from 1276 to 1277. (source)

On June 29, 1456, when what is now known as Halley's comet could be seen at night and was seen as an omen of impending disaster, Pope Calixtus III issued a papal bull against the comet, asking Christians to pray that the comet, symbolizing "the anger of God", would be fended off or be diverted solely against the Turks. (source)

At a council in Constance between 1414 and 1417, the man who called himself Pope John XXIII and is now known as Antipope John XXIII (1410–1415; not to be confused with Pope John XXIII, pope from 1958–1963) was convicted of piracy, murder, rape, and incest, and received three years in prison. (source)

By the time Rodrigo Borgia became Pope Alexander VI in 1492, he already had several children by a mistress, Vannozza Catanei. After his consecration as pope, he began an affair with a 19-year-old married woman, Giulia Farnese. As pope, he put the pursuit of wealth and worldly power ahead of the spiritual welfare of the church, which played a part in the rise of the Protestant Reformation. (source)

Pope Julius II.

In spite of the fact that the Catholic Church believed that the occult bordered on heresy, Pope Julius II set the time of his coronation in 1503 based on astrological calculations. (source)

The youngest pope since around 1400, when we have reliable dates of birth for all popes, was Pope Leo X, who was 37 when he was elected to the papacy on March 9, 1513. He was also the last person who was not a priest to become pope.

Pope Leo X (pope 1513–1521), who was a notorious spendthrift, once had to pawn his own palace furniture and silver to pay for his luxurious lifestyle and his patronage of the arts. (source)

Legend has it that Clement VII, pope from 1523 to 1534, was so fond of mushrooms that he made it illegal for anyone else to eat those growing in the Papal States, so that there would never be a shortage for his own table. He died in 1534 from eating a poisonous death cap mushroom. (source)

The papacy of Pope Urban VII lasted for just 12 days (September 15–27, 1590), the shortest ever excluding that of Stephen II. One of the few things that he did during his reign was to ban the use of tobacco "in the porchway of or inside a church, whether it be by chewing it, smoking it with a pipe or sniffing it in powdered form through the nose." Excommunication was the punishment for breaking this rule. This ban lasted until the reign of Pope Benedict XIII in 1724. (source)

Pope John XXIII had served as a sergeant in the Italian army during World War I. (source)

Pope Pius XI (1922–1939) was a mountaineer in his youth, making ascents of several mountains in the Alps including Monte Rosa and Mont Blanc. (source)

The American Institute of Management, which ranks major industries in terms of management performance, evaluated the Roman Catholic Church in 1956, awarding Pope Pius XII and his bishops 8,800 points out of a possible 10,000. Four years later, under Pope John XXIII, the AIM's "management excellence rating" climbed to 9,010 points, ranking the Church among the best-managed institutions in the world. (source)

When Pope Paul VI (1964–1978) made a pilgrimage to the Holy Land in 1964, he was the first reigning pope in over 150 years to travel outside of Italy. In contrast, Pope John Paul II (1978–2005) visited 129 countries during his reign. (source)

There were 124 popes (in addition to around 23 "anti-popes") in the second millennium (1001–2000). Even including two canonized in 2014, only seven of these have been canonized as saints. (source)

Pope John Paul II canonized more saints than all of his predecessors combined.