Cobalt‐rich manganese deposits on Pacific seamounts have become a focal point as one of the marine mineral resources with the most potential for the twenty‐first century. Extent and distribution of the deposits have not yet been well determined. Conventional survey techniques have estimated the distribution of the deposits by the surface coverage of nodules and crusts based only on visual observations. In order to determine more clearly the distribution characteristics of the deposits, a large‐diameter gravity coring system was developed and used during survey cruises of the research vesselR/V Hakurei‐maru No.2 in 1991 and 1991 Samples and photos obtained using the system have provided significant data and information for investigation of the economic potential and processes of formation of these deposits. Analysis of the cores provided important new findings on crust thickness, buried deposits, and debris flow‐like deposition. Based on the initial experience with the large‐diameter corer, some corer design improvements are presented.