Iranian archaeologists, working on the massive Chogha Zanbil ziggurat project south of Iran, plan to study the way ancient Iranians  built underground mausoleums 3,500 years ago.

The burial “palace” complex is consisted of five mausoleums, but the vault of one of them has collapsed while others are intriguingly intact. “Identifying the construction methods and materials used in building these tombs requires a special study,” said Hamid Fadaei, an archaeologist with the ziggurat project. “It is very critical for us to know how these mausoleums have survived over 3,500 years. We are going to analyze the gesso-covered walls.”

A conservationist expert also noted the construction materials had a very low level of impurity, boosting the strength of the whole structure. “They had laid bricks with specially-made mortar which has a natural waterproof insulation,” added Kazem Borhani.

He said the mausoleums are, nevertheless, vulnerable to the high level of humidity in the area and such birds and bats.

Chogha Zanbil is situated in southwest Iran about 40 km southeast of the ancient city of Susa. It was built on a plateau above the banks of the Dez River. The complex consists of a magnificent ziggurat (the largest structure of its kind in Iran), temples, and three palaces. The site was added to UNESCO's World Heritage List in 1979.

Its ancient name is Dur-Untash, which means the castle or the city of Untash. In the 13th century B.C., King Untash Napirisha founded an entirely new city. Its size and splendor was intended to honor the gods and to manifest the power of the monarch. At the center of the city, a ziggurat was built of which two floors still exist. It was surrounded by a wall, which is the inner wall of three concentric walls in Dur Untash. Between the inner wall and the middle wall several temples belonging to different Elamite divinities were built. The outer city wall was about 4 km long enclosing an area of approximately 100 hectares. The royal quarter was situated adjacent to a major city gate some 450 meters east of the ziggurat. In this area, a group of three major buildings with large courts surrounded by lengthy halls and rooms were excavated. Beneath one of theses buildings (Palace I), five underground tombs were found similar to those of Haft Tappeh (Kabnak). The tombs in Chogha Zanbil however were of a much more monumental dimension.

The building materials in Chogha Zanbil are mainly mud bricks and occasionally baked bricks. The monuments were well built and beautifully decorated with glazed baked bricks, gypsum, ornaments of faience and glass. Thousands of baked bricks bearing inscriptions with Elamite cuneiform characters were all inscribed by hand, ornamenting the most important buildings. Glazed terracotta statues such as bulls and winged griffins guarded the entrances to the ziggurat. Near the temples of Kiririsha and Hishmitik-Ruhuratir, kilns were found that probably were used for the production of baked bricks and decoration materials. The ziggurat, it is believed, was built in two stages and in the second phase took its multi-layered form.