Unsaturated tuff beneath Yucca Mountain, Nevada, is being evaluated by the U.S. Department of Energy as a host rock for a potential repository for high-level radioactive waste. As part of the Nevada Nuclear Waste Storage Investigations Project of the U.S. Department of Energy, the U.S. Geological Survey has been conducting hydrologic, geologic and geophysical investigations at Yucca Mountain and the surrounding region to provide data for evaluation of the potential suitability of the site. Hydrologic investigations of the unsaturated zone at this site began in 1982. A 17.5-inch- (44.5-centimeter) diameter borehole (USW UZ-1) was drilled by the reverse-air vacuum-drilling technique to a depth of 1269 feet (387 meters). This borehole was instrumented at 33 depth levels. At 15 of the levels, three well screens were embedded in coarse-sand columns. The sand columns were isolated from each other by thin layers of bentonite, columns of silica flour, and isolation plugs consisting of expansive cement. Thermocouple psychrometers and pressure transducers were installed within the screens and connected to the data-acquisition system at the land surface through thermocouple and logging cables. Two of the screens at each level were equipped with access tubes to allow collection of pore-gas samples. In addition to these instruments, 18 heat-dissipation probes were installed within the columns of silica flour, some of which also had thermocouple psychrometers. After more than two years of monitoring, the majority of the instruments were still functioning and producing reasonable data. A slow recovery from the disturbed state to natural conditions was detected during the first 90 days of monitoring; this recovery was probably a result of the large diameter of the borehole. Preliminary results indicated that suction pressures for the welded units ranged from −2 to −15 bars (−0.2 to −1.5 mega pascals). Some agreement existed between data from psychrometers and from heat-dissipation probes, except where silica flour was not in equilibrium with the formation. Water fluxes estimated in the matrix of the repository host-rock unit range from 4 × 10−3 to 2 × 10−2 inches per year (0.1 to 0.5 millimeter per year) using matric-potential distribution, and from −1 × 10−3 to −2 × 10−3 inches per year (−0.025 to −0.05 millimeter per year) using geothermal gradient. Responses to short-term barometric fluctuations were detected to a maximum depth of about 300 feet (91 meters) in the borehole. Below this depth, only long-term barometric fluctuations were detectable. Equivalent effective hydraulic conductivity, estimated from air permeability, ranged from 0.6 to 2 feet per day (0.2 to 0.6 meters per day).