An architectural love letter, this massive marble temple in northern India is one of the most recognizable structures on the planet. It was built in the first half of the 17th century by Mughal emperor Shah Jahan to hold the body of his beloved third wife, Mumtaz Mahal (the building is now a mausoleum for both). The construction took more than 22 years to complete, requiring as many as 20,000 workers. Some skilled artisans came from as far as Constantinople (today, Istanbul), and about 1,000 elephants were used to transport materials. Today, vehicles that emit pollution are not allowed within a mile of the structure, so be prepared to walk or hire a battery-powered vehicle called a tuk tuk.