The longest continuous excavation of caves in ancient China is the Mogao Caves, also known as the Thousand Buddha Grottoes. These caves are located in Dunhuang, a city in the Gansu province of northwest China. The Mogao Caves were first carved out of sandstone cliffs in the fourth century, and over the centuries, they were expanded and decorated with thousands of Buddhist statues, frescoes, and manuscripts. The excavation of the caves continued for over a thousand years, until the 14th century. The Mogao Caves are considered one of the most important sites of Buddhist art and culture in the world, and they have been designated a UNESCO World Heritage Site.