Valles Caldera 1 (VC-1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC-1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphic information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, and to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856-m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC-1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480-, 540-, and 845-m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.