Constructed between 1631 and 1648 on the banks of Agra's Jamuna River, the Taj Mahal was built for the Mughal emperor Shah Jahan in memory of his third wife, Mumtaz Mahal, who died giving birth. It is said that it took 20,000 stone carvers, masons and artists from across India, Turkey and Iraq, to design and build the stunning white marble monument, reports National Geographic.

The mausoleaum was designated aUNESCO World Heritage sitein 1983, and for years it has remained one of the most iconic monuments in India. It is one of the most popular tourist destinations in the world,receivingmore than three million visitors each year, according to National Geographic. But since the 1970s, visitors have noticed that its marble walls have been losing their pearly white color, tunring instead a yellowish-brown. The cause of this discoloration, reports Science Magazine, is pollution.

(MORE: 50 Unforgettable Places to See in India)

According to an article published by researchers from India and the Georgia Institute of Technology, the pollutants discoloring the Taj Mahal are airborne carbon particles, likely coming from the burning fossil fuels, cooking smoke, dust and frequent fires that are set to burn garbage and animal waste. Agriculture and road traffic could also be a cause. These carbon particles absorb ultraviolet light, explains Science magazine, turning them a yellow-brown color that stains the white marble walls of the monument. The carbon particles are also water insoluable, which is why water (and rain) doesn't wash away the dirty-looking color.

Currently, the only way effective way to clean the Taj Mahal's yellowish-brown exterior and return it to its original white color is by painstakingly applying clay on the dome’s exterior, then peeling it off. Reducing vehicle emissions and the burning of waste could also prevent additional discoloration, according to study author and Georgia Tech researcher Mike Bergin.

(For more Travel, Follow us on Facebook)