The long-lasting history of geological formation of Halong Bay is known with more than 500 million years of changes between mountain growing, marine degradation, mountain sinking, and encroaching sea. Halong used to be a very deep sea during Odovic – Silua era, shallow sea during Cacbon – Pecmi era, and coastal area during Plaogen – Neogen era. Not only that, contributing on the present natural wonder Halong Bay was the result of a magnificent ancient marine limestone accumulation, which reached above 1,000 meters 340-240 million years ago, and the Karst erosion period of more than 20 million years.