Hydrothermal fluid flow can be visualized by three-dimensional (3-D) kriging and computer contouring of oxygen isotope data on propylitized andesites in the Comstock Lode mining district. This data set consists of δ¹⁸O results for 327 samples from surface outcrops, drill cores, and mine workings in a 3.5 km × 1.7 km × 1.0 km block in the hanging wall of the Comstock fault encompassing the region where the most important ore bodies were discovered. The δ¹⁸O contours deep in the hanging wall are not simply parallel to the Comstock fault or to the boundary of the Davidson granodiorite stock as would be expected for a hydrothermal convection system dominated by unicellular flow. Instead, the subsurface δ¹⁸O values exhibit a high degree of symmetry about a vertical plane directed radially toward the stock, with isotopic contours being predominantly perpendicular to the Comstock fault and the eastern contact of the Davidson granodiorite. A vertical cross section along the Comstock Lode shows a low δ¹⁸O core with two symmetrical zones of higher δ¹⁸O values to either side where the contours are steeply inclined, which we infer reflects downwelling of fluids shifted by water-rock exchange or steam loss. This 3-D visualization of δ¹⁸O values provides the first evidence that longitudinal convective rolls can be significant in geologic environments, in this case within a zone of high temperature gradients and permeability near the Comstock fault and the Davidson granodiorite stock.