Ancient Jetavanaramaya, Sri Lanka

At the time of its completion in the 3rd century, this sacred resting place for Buddhist monks was one of the tallest structures in the ancient world. Constructed with more than 93 million baked bricks, the 400-foot-tall stupa remains one of the world’s most voluminous structures, spanning some 2.5 million square feet. Said to house relics like a sash or belt tied by the Buddha, at one point an estimated 10,000 monks may have resided within its walls. Located in the World Heritage Site of the ancient city of Anuradhapura, the Jetavanaramaya and the rest of the city were abandoned in 993 after being overrun by invaders from southern India. The city and the Jetavanaramaya were overgrown by the jungle and remained hidden for hundreds of years before the British rediscovered the sacred site in the mid-19th century. [Source]