Ha Long means Descending Dragon. The story is that during a crucial battle with the Chinese, the gods intervened and sent two dragons down to help. The islands in Ha Long bay are supposed to be the jewels the dragons spit out to stop the invading armada. It was the site of one of the most famous battles of history.
I went in for a couple of days, including a kayak trip and a cave tour. The kayaking was the best part.