Abstract: Benefit from the ability of displaying fine topography features of multi-beam sounding system, a special seabed landform drew much attention of the researchers during the investigation at the north Yellow Sea. This seabed landform was a kind of micro ring depression quite different from the submarine depressions reported previously. It developed at the plain seafloor in the water depth of 55 m to the south of the Changshan Archipelago, circular or irregular ellipse shape, with shallow grooves along the edge, protrusion or depression in the central. Each depression has different size, most of which has a major axis of about 2 km, with the shallow groove along the edge of 200~300 m wide and 0.25~2 m depth. Several micro depressions arranged in a row, with the regional trend of NW—SE or NE—SW.
The topography form and features of the micro ring depressions were described in detail in this paper based on the multi-beam survey data. They were divided it into three types with different distributions and morphologies. TypeⅠdistributed at the northeast of the area, circular or elliptical in plan view, with protrusion in the central. TypeⅡdistributed at the southeast of the area, to the north of TypeⅠarea, elliptical or elongated in plain view with depression in the central. Type Ⅲ distributed at the southwest of the area, curved in plain view, and much shorter and shallower than the previous ones. Given the circular or ellipse morphology, the annular micro depression may relate to the sub-bottom gas leakage, which can produce pockmarks with circular or elliptical shape. Meanwhile, shallow gas leakage produced by sub-bottom profile records and the deposit of peat discovered in boreholes 8 km south to the study area can support this opinion.
Therefore, it is possible that the micro ring depressions were formed by sub-bottom shallow gas leakage, and be transformed by bottom current and submarine slumping. If the suggestion can be confirmed, it will be great significance to the study of environmental evolution of the northern Yellow Sea and will provide new inspiration to the oil and gas exploration in this area.

Key words: ring depression, shallow gas, pockmark, the north Yellow Sea