Historical City of Yazd is an important centre of Persian architecture. Because of its climate, it has one of the largest networks of qanats in the world, and Yazdi qanat makers are considered the most skilled in Iran. To deal with the extremely hot summers, many old buildings in Yazd have magnificent wind towers and large underground areas. The city is also home to prime examples of yakhchals, which were used to store ice retrieved from glaciers in the nearby mountains. Yazd is also one of the largest cities built almost entirely out of adobe. Yazd's heritage as a center of Zoroastrianism is also important. There is a Tower of Silence on the outskirts, and the city has an ateshkadeh which holds a fire that has been kept alight continuously since 470 AD. Zoroastrians make up a significant minority of the population. Built in 12th century and still in use, Jame Mosque of Yazd is an example of the finest Persian mosaics and excellent architecture. Its minarets are the highest in the country. Historical city of Yazd has been registered as a World Heritage Site by UNESCO.