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From Wikipedia:
The Tower in the central part of the city reaches about 55 metres (180.45 ft) (including the height of the platform), it is the world's tallest brick tower, and is almost 1000 years old. The tomb, built of baked brick, is an enormous cylinder capped by a conical roof. The circular plan, broken by 10 flanges, is 17 m in diameter, and with 3 metre thick walls. Also, the Tower was built on such a scientific and mathematical design that at the front of the Tower, at an external circle, one can hear one's reflected echo.

Built in 1006 AD upon the orders of the Ziyarid Amir Shams ol-Ma'āli Qabus ibn Wushmgir (شمس المعالي قابوس بن وشمگير), it is located 3 km north of the ancient city of Jorjan, from where the Ziyarid dynasty ruled. Legend has it that the Sultan's body was put in a glass coffin and was suspended from the ceiling of the tower. The interiors contain the earliest examples of Muqarnas decorative styles.

A Kufic inscription at the bottom of the tower reads in Arabic:

هذا القصر العالي – لامير شمس المعالي – الامير قابوس ابن وشمگير – امر به بنائه في حياته – سنه سبع و تسعين – و ثلثمائه قمريه و سنه خمس و سبعين و ثلثمائه شمسيه

"This great monument, was built for Amir Shams ul-Ma'ali, Amir Qabus ibn Wushmgir, by his command, during his life, in the year 397 After Hijrah of the lunar calendar, and year 375 of the solar calendar"
The inscription verifies that the tower was built as a tomb for the named Ziyarid ruler.

Iran plans to submit the tower to UNESCO for registration as a World Heritage Site, for the tower's upcoming 1000th anniversary.

Gonbad-e Qābus or Gonbad-e Kāvus (also transcribed Gonbad-e Ghābus or Gonbad-i Ghāboos) is a city in the province of Golestan in Iran. The city tower was made with brick representing a special art related to the 4th century. This tower which is on a dome of 15 hight is located in the one of the principal park of Gonbad Kavoos city