The most remarkable geological events of Hạ Long Bay’s history in the last 1,000 years include the advance of the sea, the raising of the bay area and the strong erosion that has formed coral and pure blue and heavily salted water. This process of erosion by seawater has deeply engraved the stone, contributing to its fantastic beauty. Present-day Hạ Long Bay is the result of this long process of geological evolution that has been influenced by so many factors. Some of the most remarkable are: the formation of the limestone layer more than 1,000 m thick during the Carboniferous and Permian periods (240 to 340 million years ago); and the development of the Hạ Long depression during the Neogene period (10 to 26 million years ago). The erosion process forming the limestone plain was most active in the Quaternary Pleistocene epoch (11,000 to 2 million years ago). It is because of all these factors that tourists now visiting Hạ Long Bay are not only treated to one of the true wonders of the world, but also to a precious geological museum that has been naturally preserved in the open air for the last 300 million years. At the beginning of the Cambrian era (500 to 570 million years ago), the area, which now forms Hạ Long Bay, was basically mainland, submitted to a process of rain erosion. At the end of the period, it was flooded, commencing the existence of Hạ Long Bay. During the Odovic and Silurian periods (400 to 500 million years ago), the area of north-east Vietnam was basically a deep sea, submitted to the constant activity of tectonic plates. At the end of the Silurian period, it underwent a phase of inverse-motion that created mountains deep under the water. From the end of this period and throughout the whole Devonian period (340 to 420 million years ago), the area was subjected to powerful forces of erosion from the hot and dry climate. At this point, Hạ Long was part of a wide mainland that comprised most of today’s East Sea and Chinese continental shelf. Due to tectonic activity, the Hạ Long area and the entire north-east region were raised from the depths at the end of the Devonian period. In the later Carboniferous and Permian periods (240 to 340 million years ago), a shallow and warm sea reformed, which existed for approximately 100 million years. It created two kinds of limestone: the Cát Bà layer of the early Carboniferous period (450 m thick); and the Quang Hanh layer of the middle Carboniferous and the early Permian period (750 m thick). These two layers constitute the majority of the islands of the Bay. Passing into the early periods of the Contemporary era (67 million years ago), Hạ Long Bay existed in the environment of a high mountainous mainland due to the influence of strong mountain-forming phases. The middle of the Paleocene period saw these motions remain continuous and stable, while strong processes of erosion began, and after millions of years, a form of semi-highland topography took shape. The continuation of this erosion has progressively cut the highlands into blocks with altitudes similarrr to today’s mountains. Into to the Quaternary era, the process of erosion began dissolving the limestone-rich region of Hạ Long. The islands of today’s Hạ Long Bay are basically remnants of these mountains flooded during the early Holocene period. Rainwater flowed into crevices in the limestone that had formed from tectonic activity. This steady erosion constantly widened the cracks, eventually creating today’s formations. The middle and late Pleistocene epoch (11,000 to 70,000 years ago) marks the period when the famous caves and grottoes of the area formed. The Holocene period (from 7,000 to 11,000 years ago) is notable for the advance of the sea. This movement reached its peak 4,000 to 7,000 years ago and forming today’s Hạ Long Bay. After that, 3,000 to 4,000 years ago, with the sea in a steady process of recession, Hạ Long culture began to develop. At the beginning of the late Holocene epoch, the level of the water once again increased, forming a marshy floor of canals and streams, and creating the water marks that can be seen on the stone cliffs of today.