Why the ancient Dwarika town submerged in the sea when Lord Krishna left the earth?

There was no significance of Dwaraka and Bagavat Gita after the death of Krishna. That is proved when Arjuna died on the streets while he run towards the southern direction on afraid of the coming of 'kalikala'. If that was the fate of Arjuna who could hear the advice of Krishna directly, what will be ours?