One of the primary goals of South Korea’s second Ulleung Basin Gas Hydrate Expedition (UBGH2) was to examine the geotechnical properties of the marine sediment associated with methane gas hydrate occurrences found offshore of eastern Korea in the Ulleung Basin, East Sea. Methane gas hydrate is a naturally occurring crystalline solid that sequesters methane in individual molecular cages formed by a lattice of water molecules. During UBGH2, concentrated gas hydrate was found in two sedimentary environments: gas hydrate was found in thin, coarse-grained sediment layers interbedded with fine-grained sediment (fines, such as clays and muds) and as veins of essentially pure gas hydrate within predominantly fine-grained sediment. Methane gas hydrate is a potential energy resource, but the technical and economic viability of methane extraction from gas hydrate, in either of these marine environments associated with fine-grained sediments, is unknown as of 2022. This U.S. Geological Survey dataset provides insight into the reaction of diatomaceous fine-grained sediment particles to the pore water freshening that occurs when gas hydrate dissociates. To extract methane from gas hydrate, a “production” well is drilled down into the gas hydrate-bearing reservoir. The gas hydrate reservoir can be depressurized by drawing pore water out of the sediment through the production well to reduce the reservoir’s pore pressure. As the pore pressure falls below the gas hydrate stability limit, the solid gas hydrate breaks down, releasing gas and water, which then migrate toward the production well for collection. Fine-grained sediment can be a problem when extracting methane from gas hydrate because they can become resuspended in the flow of fluid and gas toward the production well. As these fine-grained particles move, they can cluster and subsequently clog pore throats in the sediment, reducing permeability, which controls how easily methane can flow toward the extraction well. The type of fine-grained sediment particle, and the chemistry of the surrounding pore water are the two main factors that determine the cluster structure (the size and fabric of the cluster), and how fast those clusters form and settle. Fine-grained particles interact with each other primarily in response to electrical forces, so changes in pore water chemistry can substantially alter how those forces are transferred between particles. In marine systems, in-situ pore water is an electrically conductive brine. As gas hydrate dissociates, however, fresh water is released along with the methane, making the pore water less conductive. In this study, fine-grained sediment samples from four UBGH2 sites are examined to better understand how the high diatom content (~22-45% by volume) of the sediment contributes to the sediment clustering and settling rate behavior. The term diatom refers here to the silica-based skeletal remains of microalgae. Diatom skeletons and skeleton fragments become buried in marine sediment when the microalgae die. Their presence can alter the clustering and settling rate of the sediment because diatoms have a lower density than most fine-grained sediment particles and can be several times larger than the typical sediment grain sizes found in the specimens studied. Diatoms can be up to 200 micrometers across, whereas the median grain size for the samples is about 10 micrometers. Specimens from the UBGH2 expedition were observed during sedimentation (settling) tests in pore fluids of differing chemistry. The results of the observations indicate the fine-grained UBGH2 sediments follow the expected behavior for diatoms in that they are extremely sensitive to the presence of low salinity levels. Even the freshening of pore water as a result of the dissociation of adjacent gas hydrate is not likely to increase the tendency of these fine-grained sediments to resuspend during the depressurization of reservoirs due to the diatom sensitivity even to low salinities.