Determination of the natural radiation environment should receive high priority during screening, characterization, construction, and operational phases of underground facilities. Radon-222, and Radon-220 (Thoron) occur widely throughout the earth's crust, and in terms of human exposure to natural radiation, the best materials for siting deep excavations are limestones, gabbros, and serpentinites (and their metamorphic equivalents) all of which have low levels of uranium concentration and radon escape. Radon daughters are natural radioactive substances whose concentration may be anomalously high in parts of some deep excavations, particularly in granitic rocks. Anomalous accumulations of radioactive minerals generate radon gas above-background. In deep unlined excavations, radon gas can contaminate the air as it is easily absorbed onto dust particles, soot (from motors), and water droplets as the gas emanates from rock faces, broken rock, faults, joints, fractures, and underground waters entering the excavation. The geochemical behavior of daughter isotopes in the radioactive decay series of uranium and thorium is generally not well understood. Considerable work is needed on ground water geochemistry and the geochemical behavior of radon precursors in relation to anomalous underground concentrations of radon.