Taq Kasra

Taq Kasra, also called the Archway of Ctesiphon, is located about 35 kilometers (22 miles) south of Baghdad. It is the only above-ground structure that remains of Ctesiphon, an ancient city that acted as the royal capital of the Persian Empire, notably during the Parthian and Sasanian periods, until the Muslim conquest of the region in the seventh century. 

While there is some debate as to the year of its construction, the archway is at least 1,400 years old. It was part of the city's palace complex, possibly the ceiling of the throne room itself (although it could have also served as a portico). Rising to 37 meters (121 feet) in height and stretching a meter (3 feet) thick at its apex and 7 meters (23 feet) thick at its supporting base, it's considered to be the largest single-span brick vault arch in the world.

Some restoration was completed under Saddam Hussein in the 1980s, but progress was halted upon the beginning of the Gulf War. While restoration was picked up again in the years since and completed in 2017, part of the arch collapsed just two years later. There is no word yet regarding further restoration or preservation plans on the site.