Density snapshots for Model QA10x2 (logarithmic color scale) at t/ t _orb = 0, 0.1, and 0.2. The initial single-temperature gas disk (a) evolves rapidly via thermal instability into a configuration with midplane cold cloudlets sandwiched by outer layers of warm gas. In (b), the first SN explosions occur in dense clouds near x = −200 pc produced by mergers and self-gravitating contraction of smaller clouds. Subsequent SN explosions disperse the dense clouds and drive the disk into a turbulent state (c), in which filamentary structures of cold gas are found at all heights.