The Persian Gulf was a very big water tank before millions of years overflowed over the Arabian peninsula in the south and Iran in the west and not like nowadays. There was a lot of salt left there when the water vanished and the see shores tumbled. The salt layers were encrusted with sediments that arrived from the rocks by the water from the rain and the layer of sediments has gotten thick over time. After that, they became dense and weighed down on salt layers.
Nowhere else in the world such cumulative salt domes can be seen, according to UNESCO. The site is not yet a World Heritage Site but is being considered for inclusion.