Short answers, pulled from the story.
The original Pantheon was commissioned by Marcus Vipsanius Agrippa on his own property in the Campus Martius between 29 and 19 BC. After it burned down twice, the present building was ordered by the emperor Hadrian and probably dedicated around AD 126, though Hadrian left Agrippa's original inscription on the new facade. Some scholars, notably Lise Hetland, argue that construction actually began under Trajan in 114.
The Pantheon's dome spans 43.3 metres across, and the interior height from floor to the oculus is the same 43.3 metres. It remains the world's largest unreinforced concrete dome, a record it has held for nearly two thousand years. It is also the only surviving ancient masonry dome that has never required tie-rods, chains, or retrofitted banding to prevent collapse.
The Byzantine emperor Phocas gave the Pantheon to Pope Boniface IV in 609, and Boniface consecrated it as a church dedicated to St. Mary and the Martyrs on the 13th of May 609. Twenty-eight cartloads of martyrs' relics were said to have been brought from the catacombs and placed beneath the high altar at that time.
Notable figures buried in the Pantheon include the painter Raphael, the painter Annibale Carracci, the composer Arcangelo Corelli, and the architect Baldassare Peruzzi. Two Italian kings, Vittorio Emanuele II and Umberto I, and Umberto's Queen Consort, Margherita, are also interred there.
Pope Urban VIII, who reigned from 1623 to 1644, ordered the massive bronze trusses above the portico removed and melted down. The archaeologist Carlo Fea established from the Pope's own accounts that about 90% of the bronze was cast into cannon for the fortification of Castel Sant'Angelo. The event inspired the famous satirical phrase: quod non fecerunt barbari fecerunt Barberini, meaning what the barbarians did not do the Barberinis did.
The Pantheon has been one of the most imitated buildings in Western architecture since at least the Renaissance. Filippo Brunelleschi studied it before building his 42-metre dome of Santa Maria del Fiore in Florence, completed in 1436. Later examples include St. Hedwig's Cathedral in Berlin (1747-73), the Rotunda at the University of Virginia, and the church of Santa Maria Assunta in Ariccia (1664) by Gian Lorenzo Bernini. Thomas Jefferson drew on Andrea Palladio's depictions of the Pantheon when designing American buildings.