The subsurface mooring, EC1, is located at a depth of 2,300 m in the middle of the Ulleung Interplain Gap (UIG) between Dokdo and Ulleung Island in the southwestern East Sea (Japan Sea). The mooring was initially deployed at EC1 in 1996 to quantify the temporal variability of deep flows, as the UIG is the only deep channel through which seawater below 1,500 m is exchanged between the Ulleung and Japan Basins. Since then, long and continuous time-series data have been collected at this mooring with a typical service period of one year to observe the temporal variability of essential ocean variables consistently. New sensors to measure water properties, as well as deep and shallow flows, have been inserted; these include Conductivity-Temperature-Depth and Dissolved Oxygen sensors. Previously published data from 1996 to 2017 were corrected to improve their quality and scientific applicability. Recently collected data from 2017 to 2020 were appended after quality control and assurance.