Discoveries from multi-beam sonar mapping and seismic-reflection surveys of Yellowstone Lake provided new insight into the recent geologic forces that have shaped this large, high-altitude lake at the active front of the Yellowstone hot spot. The Yellowstone Plateau is strongly affected by young (<2 m.y.), large-volume (>7000 km³) silicic volcanism, active tectonism, and accompanying uplift. Mapping Yellowstone Lake has enhanced our understanding of the extent of postcaldera-collapse volcanism and active hydrothermal processes occurring above a large magma chamber not only in the lake but on the Yellowstone Plateau. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to tectonic, volcanic, hydrothermal, glacial, and sedimentary processes that are similar to those observed on land within the Yellowstone Caldera. Detailed bathymetric, seismic-reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake morphology and localization of hydrothermal activity in the northern, West Thumb, and central lake basins. These observations have led us to investigate how on-land relationships influence the location of hydrothermal areas in the Yellowstone Caldera. We conclude the locations of many thermal features in the Yellowstone Caldera are directly influenced by the distribution of the rhyolitic lava flows.