This historical city came into news when India's National Institute of Ocean Technology (NIOT) discovered this historical city Gulf of Khambhat ~30-40 meters under water.
The first story is about a king called Raivata in Bhagavata Purana.
There was a king called Raivata. Raivata built a great city called “Kusasthali” in the middle of ocean. This city went under ocean after some time and again came out when oceans receded.
This Raivata had 100 sons and one daughter, Revati. He wanted to find a good groom for his daughter Revathi. In that search he and Revathi went all the way to Brahma’s world (Brahmaloka).
At that time Brahma was having a music program in his royal assembly. Raivata waited and approached Brahma after the program and requested Brahma to select a good groom for his daughter.
Brahma smiled at Reivata and told him that nearly 27 Mahayugas passed since they started their journey from Earth and none of his descendants are alive now.
Brahma told him that there is one Balarama living on earth now and would be a good candidate for Revati to marry. This Balarama is brother of Sri Krishna who built Dvārakā.
This brings us to the second reference of Dvārakā in Mahabharata.
After the death of Kamsa in the hands of Sri Krishna & Balarama, the Yadavas came under multiple invasions by Jarasamdha, King of Magadha & emperor of Bharat. To avoid this continuous state of war, Sri Krishna led migration of Yadavas to west coast of India and setup Anarta kingdom with Dvārakā as its capital in the middle of ocean.
This Dvārakā went under sea again after Mahabharata War.
Following is a scientific estimation of historical sea levels. The second graph shows historical sea levels from present times to 140,000 years in the past.
As we can see in this image, the sea levels were below 30-40m between ~7000 and ~75000 years before present indicating that the region of Dvārakā was above seal level in this period.
Since Kushasthali went below water after Raivataka, the earliest this could have happened was during the previous ocean level raise ~80,000 years ago.
This observation fits nicely with a new ground breaking research done by SriNilesh Nilkanth Oak - When Did The Mahabharata War Happen?: The Mystery of Arundhati that dates Mahabharata war to be at 5561BC (~7500 Before Present)
Following the bread crumbs:
This raises an interesting question. What about the 27 Mahayugas Brahma was talking about?