The cross section through the ground-state potential energy surface of ozone which contains the open minimum, the ring minimum, and the ring-opening reaction path, including the ring-opening transition state, is determined through full-valence-space multiconfiguration self-consistent-field calculations. It is shown that, at a point on the ridge separating the open-structure basin from the ring-structure basin in C2v symmetry, very close to the transition state, the ground-state surface connects with the lowest excited state surface of the same symmetry (1A1). This point is part of an intersection seam between these two 1A′ surfaces in Cs symmetry. It is furthermore found that the upper state has its equilibrium structure very close to the transition state of the lower state. The quantitative data of all critical points are calculated. In addition, a ground-state potential energy surface cross section describing the detachment of an oxygen atom is determined. For several other states, C2v constrained and bond-length-optimized energy curves E(φ) are also reported.