The results of a seismic refraction survey at the Idaho National Engineering Laboratory along the northwest boundary of the Snake River Plain show that velocities of the volcanic rocks beneath the Plain increase from 1.5 km/s at the surface to 5.2--5.5 km/s at depths between approximately 2200 and 2500 m. An exploration well in the area (INEL-1) encountered Cenozoic volcanic rock with some interbedded sediments to a depth of 2460 m underlain by 700 m of a rhyodacite porphyry. The refraction data demonstrate a nearly vertical faultlike discontinuity of about 1000-m displacement beneath the plain, About 1.8 km from the southeast flank of the Arco Hills (Lost River Range) Whether this discontinuity represents the northwest flank of the Snake River Plain graben, the east flank of the lost River Range fault, or a caldera wall is not known. The results further indicate that Paleozoic rocks may extend beneath the plain as far as 5.6 km southeastward from the Arco Hills.