On-going investigations of underground nuclear test environments at the Nevada Test Site (NTS) have shown that some tests are highly restricted in their hydrologic/geochemical connection with the surrounding environment. These observations are contrary to expectations, and have important implications for radionuclide fate and transport studies at the NTS. The 1973 Almendro test is a well-studied example. This test was detonated in fractured rhyolitic tuff more than 1-km underground, and >350 meters below the water table. The groundwater temperature measured within the Almendro test cavity in 1996 was still >150°C, implying that energy transfer has been dominated by conductive heat loss. The d¹⁸O value of the cavity fluid is presently shifted by +1.5 permil relative to ambient groundwater isotopic values. The lack of a correlated change in dD implies the oxygen isotope shift is related to water-rock exchange, as seen in natural geothermal waters. Dissolved inorganic carbon (DIC) in groundwater from the Almendro cavity has a d¹³C value of +27 permil, and a sample collected in the borehole above the cavity has a d¹³C value of +45 permil. In comparison, d¹³C values of groundwater from nearby environmental wells range from -1 to -11 permil. The Almendro d¹³C data are generally consistent with a methanogenic process, such as CO₂ reduction (e.g. CO₂ + 8H⁺ + 8e^- ® CH₄ + 2H₂O), which tends to enrich the residual DIC in ¹³C. Work is in progress to determine whether this process is bacterially mediated or abiotic. Available evidence suggests the permeability of the surrounding rock is a key factor governing the development of environmentally restricted conditions. Following the Almendro test, near-field water levels were depressed by 250 m relative to pre-test conditions and required more than 20 years to recover. Local hydrothermal circulation of a limited groundwater supply in the cavity region may cause the system to self-seal as secondary minerals are deposited along fractures. This work was funded by the Underground Test Area Project and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-Eng-48.