Dwarka, which is mentioned in many Hindu scriptures including the Mahabharata, was the kingdom of Sri Krishna . It is believed that the Dwarka, which is under sea now, existed somewhere around 3200 years to 9000 years ago. A documentary on underwater cities and alien visits to earth, suggests that Dwarka might have been destroyed by Aliens.

Facts – A city was discovered underwater near present day Dwarka in Gujarat . It was a very ancient settlement and busy port. Sandstones walls and streets were discovered by underwater archeological studies.
There are two versions of the destruction of Dwarka in Hindu Scriptures – the most popular is that when Krishna decided to leave the world, the city of Dwarka was submerged by sea – sea rising to eat Dwarka.
Another version has that a King named Shalva attacked Dwarka from the skies using weapons like lightning and thunder.
About Aliens destroying the city – The documentary suggests that the attack of Shalva is similar to Alien attacks. Krishna retaliated with bolts of lightning to the attack from the sky.
Note – the documentary indicates numerous theories with reference to ancient Hindu Scriptures. Only the first seven minutes of the documentary deal with Dwarka.
You can watch the documentary here – Documentary on Krishna ’s Dwarka (link not working)
You can watch the same documentary in embedded form below. (the file has been removed the particular website)
Book on Dwarka - underwater excavations